drm/i915: s/disable_lp_wm/disable_cxsr/

drm/i915: s/disable_lp_wm/disable_cxsr/

The ilk+ disable_lp_wm boolean has the exact same role as
disable_cxsr for gmch platforms. The documentation also
still talks about CxSR on ilk+ even theough the way you
control it has now change to involve toggling the LP watermarks.
Get rid of disable_lp_wm and just use disable_cxsr for ilk+
as well.

TODO: Unify even more to not have any gmch vs. ilk+
      details in high level modeset code...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240916162413.8555-7-ville.syrjala@linux.intel.com
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>