drm/i915/icl: Add register definitions for Multi Segmented gamma

Add macros to define multi segmented gamma registers

V2: Addressed Ville's comments:
    	Add gen-lable before bit definition
    Addressed Jani's comment
	- Use REG_GENMASK() and REG_BIT()
V3: Addressed Ville's comments:
    - Put comments at the end of line.
    - Change the comment at start of ICL multisegmented gamma registers.
    Added Ville's r-b

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1560321900-18318-3-git-send-email-uma.shankar@intel.com