dma-buf: add more reservation object locking wrappers

Complete the abstraction of the ww_mutex inside the reservation object.

This allows us to add more handling and debugging to the reservation
object in the future.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/320761/