drm/i915/dp: make backlight bl_power control power sequencer backlight

This lets the userspace switch off the backlight using the backlight
class sysfs bl_power file. The switch is done using the power sequencer;
the backlight PWM, and everything else, remains enabled. The display
backlight won't draw power, but for maximum power savings the encoder
needs to be switched off.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed_by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Tested_by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>