drm/i915: Add heuristic to determine better way to adjust brightness

Add heuristic to decide that AUX or PWM pin should use for
backlight brightness adjustment and modify i915 param description
to have auto, force disable, and force enable.

The heuristic to determine that using AUX pin is better than using
PWM pin is that the panel support any of the feature list here.
- Regional backlight brightness adjustment
- Backlight PWM frequency set
- More than 8 bits resolution of brightness level
- Backlight enablement via AUX and not by BL_ENABLE pin

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622190339.142671-3-puthik@chromium.org