drm/i915: Extract skl_wm_crtc_disable_noatomic()

drm/i915: Extract skl_wm_crtc_disable_noatomic()

Hoist the dbuf stuff into a separate function from
intel_crtc_disable_noatomic_complete() so that the details
are better hidden inside skl_watermark.c.

We can also skip the whole thing on pre-skl since the dbuf state
isn't actually used on those platforms. The readout path does
still fill dbuf_state->active_pipes but we'll remedy that later.

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/20250306163420.3961-6-ville.syrjala@linux.intel.com