drm/vc4: Use drm_gem_fb_prepare_fb

vc4 has switched to using drm_fb->obj[], so we can just use the helper
unchanged.

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>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625204208.5614-4-daniel.vetter@ffwll.ch