drm/i915: Make DP prepare/commit consistent with DP dpms

Make sure the sequence of operations in all three functions makes
sense:

 1) The backlight must be off unless the screen is running
 2) The link must be running to turn the eDP panel on/off
 3) The CPU eDP PLL must be running until everything is off

Signed-off-by: Keith Packard <keithp@keithp.com>