drm/amdgpu: fix colliding of preemption

what:
some os preemption path is messed up with world switch preemption

fix:
cleanup those logics so os preemption not mixed with world switch

this patch is a general fix for issues comes from SRIOV MCBP, but
there is still UMD side issues not resovlved yet, so this patch
cannot fix all world switch bug.

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