drm: vc4: Switch to use drm_gem_object reservation_object

drm: vc4: Switch to use drm_gem_object reservation_object

Now that the base struct drm_gem_object has a reservation_object, use it
and remove the private BO one.

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190202154158.10443-6-robh@kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>