drm/i915/bxt: add bxt dsi gpio element support

Request the GPIO by index through the consumer API. For now, use a quick
hack to store the already requested ones, simply because I have no idea
whether this actually works or not, and I have no way to test it.

v2 by Mika: switch *NULL* to *"panel"* when requesting gpio for MIPI/DSI
panel.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480923034-21916-1-git-send-email-mika.kahola@intel.com