drm/i915/gvt: Stop waiting whilst holding struct_mutex

For whatever reason, the gvt scheduler runs synchronously. At the very
least, lets run synchronously without holding the struct_mutex.

v2: cut'n'paste mutex_lock instead of unlock.
Replace long hold of struct_mutex with a mutex to serialise the worker
threads.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>