drm/i915: abstract i2c bit banging fallback in gmbus xfer

Choose between i2c bit banging and gmbus in a new higher level function,
and let the i2c core retry the first time we fall back to bit banging.

The i2c core imposes a timeout on -EAGAIN, but it defaults to 1 second,
and shouldn't be a problem for us.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448980166-23055-2-git-send-email-jani.nikula@intel.com