drm/radeon: properly document reloc priority mask

Instead of hard coding the value properly document
that this is an userspace interface.

No intended functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/gpu/drm/radeon/radeon_cs.c b/drivers/gpu/drm/radeon/radeon_cs.c
index ee712c1..cb12df7 100644
--- a/drivers/gpu/drm/radeon/radeon_cs.c
+++ b/drivers/gpu/drm/radeon/radeon_cs.c
@@ -132,7 +132,8 @@
 		 * the buffers used for read only, which doubles the range
 		 * to 0 to 31. 32 is reserved for the kernel driver.
 		 */
-		priority = (r->flags & 0xf) * 2 + !!r->write_domain;
+		priority = (r->flags & RADEON_RELOC_PRIO_MASK) * 2
+			   + !!r->write_domain;
 
 		/* the first reloc of an UVD job is the msg and that must be in
 		   VRAM, also but everything into VRAM on AGP cards to avoid