drm/i915: Move the dodgy pre-g4x wm stuff into i9xx_wm

drm/i915: Move the dodgy pre-g4x wm stuff into i9xx_wm

As with other watermark calculations, the dodgy pre-g4x
update_wm_{pre,post} flag calcultion would like to know
if a modeset is about to happen or not, and technically
later stages in the atomic_check() may still flag one.
In practice that shouldn't happen as we don't have dynamic
CDCLK implemented for these old platforms.

Regardless it'll be nice to move this old cruft out from
the supposedly platform agnostic plane code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240916162413.8555-6-ville.syrjala@linux.intel.com
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>