drm/i915: Parse VBT data for lspcon

Many GEN9 boards come with on-board lspcon cards.
Fot these boards, VBT configuration should properly point out
if a particular port contains lspcon device, so that driver can
initialize it properly.

This patch adds a utility function, which checks the VBT flag
for lspcon bit, and tells us if a port is configured to have a
lspcon device or not.

V2: Fixed review comments from Ville
- Do not forget PORT_D while checking lspcon for GEN9

V3: Addressed review comments from Rodrigo
- Create a HAS_LSPCON() macro for better use case handling.
- Do not dump warnings for non-gen-9 platforms, it will be noise.

V4: Rebase
V5: Rebase
V6: Pass dev_priv to HAS_LSPCON() macro

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/1476455212-27893-4-git-send-email-shashank.sharma@intel.com