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

This is to workaround guest driver hang regression after
preemption enable that gvt hasn't enabled handling of that
for guest workload. So in effect this disables preemption
for gvt context now.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
(cherry picked from commit 1603660b)