drm/i915: move VBT based DSI presence check to intel_bios.c

Hide knowledge about VBT child devices in intel_bios.c.

v2: Move port check to intel_bios.c (Sivakumar)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458125015-7931-4-git-send-email-jani.nikula@intel.com