drm/i915/gvt: use vgpu lock for active state setting

Need to align with deactivate, should only use vgpu's lock for
active state setting instead of gvt lock.

Fixes: f25a49ab ("drm/i915/gvt: Use vgpu_lock to protect per vgpu access")
Cc: Colin Xu <colin.xu@intel.com>
Reviewed-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191202070109.73924-2-zhenyuw@linux.intel.com