drm/i915: Split wm sanitize from readout

I'll need to move the wm readout to an earlier point in the
sequence (since the bw state readout will need ddb information
from the wm readout). But (at least for now) the wm sanitation
will need to stay put as it needs to also sanitize things for
any pipes/planes we disable later during the hw state takeover.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306163420.3961-16-ville.syrjala@linux.intel.com