drm/i915/bxt: BLC implementation

Enabling BLC on BXT.
Includes register definition, and new functions for BXT.

In BXT, there are 2 sets of registers for BLC. Until there is clarity
about which set would be effective, set 1 is being used.
This would have to be re-visited if there is any change or when 2 LFPs are
enabled on BXT.

This patch enables brightness change which would be effected by use of
hot-keys or sysfs entry.

TODO:- BLC implementation will have to re-visited when
1. there is clarity about which set of registers has to be used and when.
2. CDCLK frequency is changed

v2: Jani's review comments
	- Modified comment in i915_reg.h
	- Renamed register defintions
	- Removed definition of duty cycle max. Not required now and its not 64-bit.

v3:
  - Rebase on top of VLV/CHV backlight changes, in particuliar
    bxt_set_backlight() now has a different prototype (Damien)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Shankar, Uma <uma.shankar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>