Revert "drm/i915/gvt: set max priority for gvt context"

This reverts commit 11474e90.

There are issues which will block the host preemption before, instead of
disabling it use one workaround "setting max priority for gvt context"
to avoid the gvt context be preempted by the host. Now the issues have been
cleared, so revert this patch to enable host preemption.

v2:
- refine description(Zhenyu)

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>