drm/i915/icp: Add backlight Support for ICP

drm/i915/icp: Add backlight Support for ICP

ICP has two backlight controllers - similar to previous platforms like
BXT -, but we only use one controller for now, so we can just reuse
the CNP code.

v2: Remove the usage of ICP_SECOND_PPS_BACKLIGHT register.(Jani)
Reuse CNP code since it is very similar.(Ville)
v3 (from Paulo): Rebase.
v4 (from Paulo): adjust commit message (James) and comment (Rodrigo).

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180119184812.2888-1-paulo.r.zanoni@intel.com
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>