drm/amdgpu/vpe: align with mcbp changes

MCBP is decided by adev->gfx.mcbp now.
This is missed in rebase.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>