drm/amdgpu: disable mcbp if parameter zero is set

The parameter amdgpu_mcbp shall have priority against the default value
calculated from the chip version.
User could disable mcbp by setting the parameter mcbp as zero.

v2: do not trigger preemption in sw ring muxer when mcbp is disabled.

Signed-off-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>