drm/panel: ilitek-ili9881c: prepare for adding support for extra panels

There're more panels with ILI9881C controller than the Bananapi one
supported by this driver.

Extract the mode and init sequence part, to prepare the driver for
adding new panels.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200719170411.275812-2-icenowy@aosc.io