drm/amdgpu/dce11: Re-set VBLANK interrupt state when enabling a CRTC

Something (ATOM BIOS?) seems to be clobbering the LB_INTERRUPT_MASK
register while the CRTC is off, which caused e.g. glxgears or
gnome-shell to hang after a modeset.

Reviewed-and-Tested-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Sonny Jiang <sonny.jiang@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>