drm/i915/dsi: refer to gpio index instead of gpio to avoid confusion

The DSI sequence blocks contain gpio index references, not actual gpio
numbers. No functional changes.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/4a54778e56b507e8a0bd635ba02ed2a4734b00ac.1458299160.git.jani.nikula@intel.com