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