gpu: ipu-v3: dp: fix CSC handling

Initialize the flow input colorspaces to unknown and reset to that value
when the channel gets disabled. This avoids the state getting mixed up
with a previous mode.

Also keep the CSC settings for the background flow intact when disabling
the foreground flow.

Root-caused-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>