drm/ingenic: ipu: Only enable clock when needed

Instead of keeping the IPU clock enabled constantly, enable and disable
it on demand, when the IPU plane is used. That way, we won't use any
extra power when the IPU is not used.

v2: Explain the reason of this patch

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730144830.10479-4-paul@crapouillou.net