drm: msm: Switch to use drm_gem_object reservation_object

drm: msm: 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.

We can't use the drm_gem_reservation_object_wait() helper for MSM
because (in theory) msm_gem_cpu_prep() will also do some cache
maintenance on the GEM object.

Cc: David Airlie <airlied@linux.ie>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190202154158.10443-4-robh@kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>