drm/i915: Enable lspcon initialization

This patch adds initialization code for lspcon.
What we are doing here is:
	- Check if lspcon is configured in VBT for this port
	- If lspcon is configured, initialize it and configure it
          as DP port.

V2: Addressed Ville's review comments:
- Not adding AVI IF functions for LSPCON display now.
  This part will be added once the dig_port level AVI-IF series
  gets merged.

V3: Rebase
V4: Rebase
V5: Rebase
V6: Rebase

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476455212-27893-5-git-send-email-shashank.sharma@intel.com