drm/i915: do full backlight setup at enable time

We should now have all the information we need to do a full
initialization of the backlight registers.

v2: Keep QUIRK_NO_PCH_PWM_ENABLE for now (Imre).

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>