drm/i915: provide more error output when mode sets fail

If a mode set fails we may get a message from drm_crtc_helper if we're lucky,
but it won't tell us anything about *why* we failed to set a mode.  So
add a few DRM_ERRORs for the cases that shouldn't happen so we can debug
things more easily.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>