drm/amdgpu: Add to initialization of mmVCE_VCPU_CNTL register

Add a bit needed during initialization into the driver, where it is supposed
to be.  Currently, this is happening in the VCE firmware, and although
functional, this is the correct place to perform this initialization.

Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alan Harrison <Alan.Harrison@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>