drm/i915: make backlight hooks connector specific

Previously we've relied on having basically one backlight and one
backlight type per platform. This is already a bit quirky with PMIC PWM
support on VLV/CHV platforms with MIPI DSI. In the foreseeable future
we'll have at least DPCD based backlight control on eDP and DCS command
based backlight control on MIPI DSI. Backlight is becoming more and more
connector specific, so reflect this fact by making the backlight control
hooks connector specific.

This enables further work to reuse generic backlight code in
intel_panel.c while adding more specific backlight code accessed via the
hooks.

Cc: Deepak M <m.deepak@intel.com>
Cc: Yetunde Adebisi <yetundex.adebisi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Deepak M <m.deepak@intel.com>
Reviewed-by: Yetunde Adebisi <yetundex.adebisi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>