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

Port of radeon commit:
6d3759fa

v2: change radeon reference in error message

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