drm/radeon: Re-show the cursor after a modeset

Setting a mode seems to clear the cursor registers, so we need to
re-program them to make sure the cursor is visible.

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