drm: Add reservation_object to drm_gem_object

Many users of drm_gem_object embed a struct reservation_object into
their subclassed struct, so let's add one to struct drm_gem_object.
This will allow removing the reservation object from the subclasses
and removing the ->gem_prime_res_obj callback.

With the addition, add a drm_gem_reservation_object_wait() helper
function for drivers to use in wait ioctls.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190202154158.10443-2-robh@kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>