drm/i915/gvt: Wean off struct_mutex

Use the local vgpu_lock while preparing workloads to avoid taking the
obsolete i915->drm.struct_mutex

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016183902.13614-1-chris@chris-wilson.co.uk