drm/amdgpu: Disable gfx off if VCN is busy

this patch is a workaround for the gpu hang
at video begin/end time if gfx off is enabled.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>