drm/i915/dsi: use a temp variable for referencing the gpio table

The shorthand is easier. Also change the struct name. No functional
changes.

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/6572c108424a67b02367ea69cbbe00a03af9b958.1459884518.git.jani.nikula@intel.com