drm/i915: handle backlight through chip specific functions

The backlight code has grown rather hairy, not least because the
hardware registers and bits have repeatedly been shuffled around. And
this isn't expected to get any easier with new hardware. Make things
easier for our (read: my) poor brains, and split the code up into chip
specific functions.

There should be no functional changes.

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>