Following the introduction of bridge_atomic_enable in the ingenic
drm driver, the crtc is enabled between .prepare and .enable, if
it exists. Add it so the backlight is only enabled after the crtc is, to
avoid graphical issues.

As we're moving the "sleep out" command out of the init sequence
into .enable for the ABT, we need to switch the regmap cache
to REGCACHE_FLAT to be able to use regmap_set_bits, given this
panel registers are write-ony and read as 0.
Signed-off-by: Christophe Branchereau <cbranchereau@gmail.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
[pcercuei: Remove empty line after opening brace]
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321133651.291592-4-cbranchereau@gmail.com