drm/amdgpu/dce8: Move hotspot handling out of set_cursor

It's only needed in dce8 crtc_cursor_set2.

Port of radeon commit:
2e007e61

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