drm/i915/dsi: add support for DSI sequence block v2 gpio element

In sequence block v2, and only in v2, the gpio source (i.e. IOSF port)
is specified separately.

v2: initialize gpio_source to 0 and handle v1 and v2 in the same branch

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/87152feec8f921dc82502af1b29c0956b0d360bb.1458299160.git.jani.nikula@intel.com