drm/radeon: add userptr flag to limit it to anonymous memory v2

Avoid problems with writeback by limiting userptr to anonymous memory.

v2: add commit and code comments

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_gem.c b/drivers/gpu/drm/radeon/radeon_gem.c
index 993ab22..032736b 100644
--- a/drivers/gpu/drm/radeon/radeon_gem.c
+++ b/drivers/gpu/drm/radeon/radeon_gem.c
@@ -290,7 +290,8 @@
 		return -EACCES;
 
 	/* reject unknown flag values */
-	if (args->flags & ~RADEON_GEM_USERPTR_READONLY)
+	if (args->flags & ~(RADEON_GEM_USERPTR_READONLY |
+	    RADEON_GEM_USERPTR_ANONONLY))
 		return -EINVAL;
 
 	/* readonly pages not tested on older hardware */