radv: Use correct image view comparison for fast clears.

The if is actually returning true on success, enabling fast clears, so we
need to have the test succeed when the iview dimensions are right.

Fixes: d5400a5e "radv: provide a helper for comparing an image extents."
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit a777c3d7)