drm/i915/dsi: abstract VLV gpio element execution to a separate function

Prepare for future. No functional changes.

v2: Move earlier in the series. Use bool for gpio value.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[Jani: restored fixme comment while applying.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ee791fed271d7f31c34163de6c6be37d1b704ef3.1459884518.git.jani.nikula@intel.com