drm/radeon: Restrict offset for legacy display engine.

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

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>