drm/amdgpu:enable mcbp for gfx9(v2)

set bit 21 of IB.control filed to actually enable
MCBP for SRIOV
v2:
add flag for preemption enable bit for soc15 and use
this flag instead of hardcode.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>