drm/i915/bios: drop has_mipi in favor of intel_bios_is_dsi_present

Favor a single point of truth instead of duplicating the
information. The change also filters out unsupported DSI ports at this
stage, accepting only ports A and C, instead of waiting until the port
checks.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458125015-7931-6-git-send-email-jani.nikula@intel.com