drm/panel: ilitek-ili9881c: Make gpio-reset optional

Depends in how logic is connected to the board the gpio is
not stricly required.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211016102232.202119-5-michael@amarulasolutions.com