drm/radeon: remove assignment for return value

Remove unnecessary assignment for return value and have the
function return the required value directly.
Issue found by coccinelle:
@@
local idexpression ret;
expression e;
@@

-ret =
+return
     e;
-return ret;

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Wambui Karuga <wambui@karuga.xyz>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>