drm/i915: Parse DSI backlight/cabc ports.

This patch parse DSI backlight/cabc ports info from
VBT and save them inside local structure. This saved info
can be directly used while initializing DSI for different
platforms instead of parsing for each platform.

V2: Changes:
    - Typo fix in commit message.
    - Move up newly added port variables (Jani N)
    - Remove redundant initialization (Jani N)
    - Don't parse CABC ports if not supported (Jani N)
V3: Patch restructure (Suggested by Jani N)

Credits-to: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507898700-20016-1-git-send-email-madhav.chauhan@intel.com