drm: panel-orientation-quirks: Convert to use match_string() helper

The new helper returns index of the matching string in an array.
We are going to use it here.

Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503184119.22355-1-andriy.shevchenko@linux.intel.com