imx-drm: core: handling of DI clock flags to ipu_crtc_mode_set()

We do not need to track the state of the IPU DI's clock flags by having
each display bridge calling back into imx-drm-core, and then back out
into ipuv3-crtc.c.

ipuv3-crtc can instead just scan the list of encoders to retrieve their
type, and build up a picture of which types of encoders are attached.
We can then use this information to configure the IPU DI clocking mode
without any uncertainty - if we have multiple bridges connected to the
same DI, if one of them requires a synchronous DI clock, that's what we
must use.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>