drm/i915/dsi: clarify GPIO exec sequence

With the various sequence versions and pointer increments interleaved,
it's a bit hard to decipher what's going on. Add separate paths for
different sequence versions.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103201831.1037416-4-andriy.shevchenko@linux.intel.com