drm/i915: Wait for LVDS panel power sequence

During mode setting, check to make sure the panel power sequencing has
completed before doing further operations on the device. This
uncovered errors with DPMS not turning the device off as it was left locked.

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