drm/i915: Use the correct plane source width in watermark calculations

Currently we sometimes use the plane destination width, or just the
pipe src width as the plane source width in the watermark calculatons.
Use the correct thing everywhere.

v2: convert ilk cur/fbc cases too

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211209144311.3221-2-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>