Merge the bdw changes into the backlight rework branch so that we can
adapt the new code for bdw, too.  This is a bit a mess, but doing this
another way would have delayed the merging of the backlight
refactoring. Mea culpa.

As discussed with Jani on irc only do bdw-specific callbacks for the
set/get methods and bake in the only other special-case into the pch
enable function.

Conflicts:
	drivers/gpu/drm/i915/intel_panel.c

v2: Don't enable the PWM too early for bdw (Jani).

v3: Create new bdw_ functions for setup and enable - the rules change
sufficiently imo with the switch from controlling the pwm from the cpu
to controlling it completel from the pch to warrant this.

v4: Rip out unused pipe variable in bdw_enable_backlight (0-day
builder).

Tested-by: Ben Widawsky <ben@bwidawsk.net> (on bdw)
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>