drm/vgem: Switch to reservation_object_lock() helpers

For the convenience of encapsulation the reservation object's ww_mutex
was wrapped in pair of lock/unlock helpers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123095357.29514-1-chris@chris-wilson.co.uk