dma-buf: Provide wrappers for reservation's lock

Joonas complained that writing ww_mutex_lock(&resv->lock, ctx) was too
intrusive compared to reservation_object_lock(resv, ctx);

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115154642.31850-1-chris@chris-wilson.co.uk