drm/radeon: Hide cursor on CRTCs used by fbdev (v2)

Since we are now preserving the cursor across modesets, the cursor could
be left over in console if e.g. X crashed.

v2: add comment about universal plane support

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