drm/radeon: Restrict offset for legacy hardware cursor.

commit c4353016dac10133fa5d8535af83f0c4845a2915 upstream.

The hardware only takes 27 bits for the offset, so larger offsets are
truncated, and the hardware cursor shows random bits other than the intended
ones.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=46796

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>