drm/panel: ilitek-ili9881c: Read panel orientation

The panel orientation needs to parsed from a device-tree and assigned to
the panel's connector in order to make orientation property available to
userspace. That's what this patch does for ILI9881C based panels.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020153432.383845-4-john@metanate.com