drm/i915/gvt/kvmgt: dereference the pointer within lock

Though there is no issue exposed yet, it's possible that another
thread releases the entry while our trying to deref it out of the
lock. Fit it by moving the dereference within lock.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>