drm/msm: Use drm_gem_fb_prepare_fb

msm has switched over to drm_fb->obj[] a while ago already, so we can
just use the helper.

v2: Make it compile ... oops.

Cc: Eric Anholt <eric@anholt.net>
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Jeykumar Sankaran <jsanka@codeaurora.org>
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Bruce Wang <bzwang@chromium.org>
Cc: Fritz Koenig <frkoenig@google.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190625204208.5614-3-daniel.vetter@ffwll.ch