drm/i915: Let panel power sequencing hardware do its job

The panel power sequencing hardware tracks the stages of panel power
sequencing and signals when the panel is completely on or off. Instead
of blindly assuming the panel timings will work, poll the panel power
status register until it shows the correct values.

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