drm/amdgpu: disable MCBP by default

Disable MCBP(mid command buffer preemption) by default as old Mesa
hangs with it. We shall not enable the feature that breaks old usermode
driver.

Fixes: 50a7c876

 ("drm/amdgpu: enable mcbp by default on gfx9")
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>
Cc: stable@vger.kernel.org