drm/amdgpu: Disable MCBP from soc21 for SRIOV

[why]
Start from soc21, CP does not support MCBP, so disable it.

[how]
Used amgpu_mcbp flag alone instead of checking if is in SRIOV to
enable/disable MCBP.
Only set flag to enable on asic_type prior to soc21 in SRIOV.

Signed-off-by: Yiqing Yao <yiqing.yao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>