drm: fix wrong degamma/shaper TFs on NV12

For BT.709, the AMD driver-specific color management properties define
the inverse OETF for plane degamma and blend predefined TFs and the OETF
for plane shaper and CRTC regamma TFs.

By correctly setting degamma and shaper TFs to NV12 planes, we can bring
degamma and shaper+3D LUT back and remove the workaround made in [1].

[1] 3b69847 ("drm: Add workaround for NV12 scanout on SteamOS 3.6")

src/Backends/DRMBackend.cpp

@@ -70,16 +70,6 @@ gamescope::ConVar<bool> cv_drm_debug_disable_color_range( "drm_debug_disable_col
 gamescope::ConVar<bool> cv_drm_debug_disable_explicit_sync( "drm_debug_disable_explicit_sync", false, "Force disable explicit sync on the DRM backend." );
 gamescope::ConVar<bool> cv_drm_debug_disable_in_fence_fd( "drm_debug_disable_in_fence_fd", false, "Force disable IN_FENCE_FD being set to avoid over-synchronization on the DRM backend." );
 
-// HACK:
-// Workaround for AMDGPU bug on SteamOS 3.6 right now.
-// Using a Shaper or 3D LUT results in the commit failing, and we really want
-// NV12 direct scanout so we can get GFXOFF.
-// The compromise here is that colors may look diff to when we composite due to
-// lack of 3D LUT, etc.
-// TODO: Come back to me on the kernel side after figuring out what broke
-// since we moved to the upstream properites and a bunch of work happened.
-gamescope::ConVar<bool> cv_drm_hack_nv12_color_mgmt_fix( "drm_hack_nv12_color_mgmt_fix", true, "If using NV12, disable explicit degamma + shaper + 3D LUT" );
-
 namespace gamescope
 {
 	std::tuple<int32_t, int32_t, int32_t> GetKernelVersion()
@@ -2457,23 +2447,17 @@ drm_prepare_liftoff( struct drm_t *drm, const struct FrameInfo_t *frameInfo, boo
 						//
 						// Doing LINEAR/DEFAULT here introduces banding so... this is the best way.
 						// (sRGB DEGAMMA does NOT work on YUV planes!)
-						degamma_tf = AMDGPU_TRANSFER_FUNCTION_BT709_OETF;
-						shaper_tf = AMDGPU_TRANSFER_FUNCTION_BT709_INV_OETF;
+						degamma_tf = AMDGPU_TRANSFER_FUNCTION_BT709_INV_OETF;
+						shaper_tf = AMDGPU_TRANSFER_FUNCTION_BT709_OETF;
 					}
 
 					bool bUseDegamma = !cv_drm_debug_disable_degamma_tf;
-					if ( bYCbCr && cv_drm_hack_nv12_color_mgmt_fix )
-						bUseDegamma = false;
-
 					if ( bUseDegamma )
 						liftoff_layer_set_property( drm->lo_layers[ i ], "AMD_PLANE_DEGAMMA_TF", degamma_tf );
 					else
 						liftoff_layer_set_property( drm->lo_layers[ i ], "AMD_PLANE_DEGAMMA_TF", 0 );
 
 					bool bUseShaperAnd3DLUT = !cv_drm_debug_disable_shaper_and_3dlut;
-					if ( bYCbCr && cv_drm_hack_nv12_color_mgmt_fix )
-						bUseShaperAnd3DLUT = false;
-
 					if ( bUseShaperAnd3DLUT )
 					{
 						liftoff_layer_set_property( drm->lo_layers[ i ], "AMD_PLANE_SHAPER_LUT", drm->pending.shaperlut_id[ ColorSpaceToEOTFIndex( entry.layerState[i].colorspace ) ]->GetBlobValue() );