drm/amdgpu/dce10: Re-set VBLANK interrupt state when enabling a CRTC

This doesn't seem strictly necessary with Tonga right now, but that might
change with future power management enhancements.

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