gpu: ipu-v3: ipu-dc: Simplify display controller microcode setup

This cleans up the display controller microcode setup in ipu_dc_init_sync
a little bit. The microcode template words for DI0 and DI1 are properly
separated to avoid a clash when DI1 is active in interlaced mode at the
same time as DI0 in non-interlaced mode.
A comment is added to explain the meaning of the sync counter.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>