drm/i915/xe3lpd: Adjust watermark calculations

Xe3 makes a couple minor tweaks to the watermark algorithm's block count
calculations.

Bspec: 68985
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241010224311.50133-3-matthew.s.atwood@intel.com