drm/i915: Handle SKL+ WM/DDB registers next to all other plane registers

drm/i915: Handle SKL+ WM/DDB registers next to all other plane registers

Having the plane WM/DDB regitster write functions in skl_watermarks.c
is rather annoying when trying to implement DSB based plane updates.
Move them into the respective files that handle all other plane
register writes. Less places where I need to worry about the DSB
vs. MMIO decisions.

The downside is that we spread the wm struct details a bit further
afield. But if that becomes too annoying we can probably abstract
things a bit more with a few extra functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510152329.24098-17-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>