drm/radeon: Always disable RADEON_GEM_GTT_UC along with RADEON_GEM_GTT_WC

Write-combining is a CPU feature. From the GPU POV, these both simply
mean no GPU<->CPU cache coherency.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_object.c

@@ -231,7 +231,7 @@ int radeon_bo_create(struct radeon_device *rdev,
 	/* XXX: Write-combined CPU mappings of GTT seem broken on 32-bit
 	 * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
 	 */
-	bo->flags &= ~RADEON_GEM_GTT_WC;
+	bo->flags &= ~(RADEON_GEM_GTT_WC | RADEON_GEM_GTT_UC);
 #elif defined(CONFIG_X86) && !defined(CONFIG_X86_PAT)
 	/* Don't try to enable write-combining when it can't work, or things
 	 * may be slow
@@ -243,7 +243,7 @@ int radeon_bo_create(struct radeon_device *rdev,
 
 	DRM_INFO_ONCE("Please enable CONFIG_MTRR and CONFIG_X86_PAT for "
 		      "better performance thanks to write-combining\n");
-	bo->flags &= ~RADEON_GEM_GTT_WC;
+	bo->flags &= ~(RADEON_GEM_GTT_WC | RADEON_GEM_GTT_UC);
 #endif
 
 	radeon_ttm_placement_from_domain(bo, domain);