drm/amdgpu/dce: fix mask in dce_v*_0_is_in_vblank

Using the wrong mask.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Noticed-by: Hans de Ruiter <hans@keasigmadelta.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>