drm/i915: Warn when inteldrmfb fails to restore its framebuffer config

While sifting through the inteldrmfb code trying to solve #22040 I found that
the fb restore path doesn't check the return value of
drm_crtc_helper_set_config(), which seems to have all sorts of potential
failure modes. We should warn someone if one of these is triggered.

Signed-Off-By: Ben Gamari <bgamari.foss@gmail.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: hand-applied, failures are mine]
Signed-off-by: Eric Anholt <eric@anholt.net>