drm/ingenic: Fix support for JZ4780 HDMI output

We have to make sure that
- JZ_LCD_OSDC_ALPHAEN is set
- plane f0 is disabled as it's not working yet

Tested on MIPS Creator CI20 board.

Signed-off-by: Paul Boddie <paul@boddie.org.uk>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Fixes: ef2f5d0a

 ("drm/ingenic: prepare ingenic drm for later addition of JZ4780")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
[pcercuei: add proper fixes commit, slightly reword commit description]
Link: https://patchwork.freedesktop.org/patch/msgid/9d3a2000d2bb014f1afb0613537bdc523202135d.1644681054.git.hns@goldelico.com