drm: etnaviv: 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: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: etnaviv@lists.freedesktop.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190202154158.10443-3-robh@kernel.org


Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>