drm/i915/gen9: implement missing case for SKL watermarks calculation

This should affect linear and X tiled planes on really small htotal
cases. It doesn't seem to be a very feasible case, but let's implement
it since it's on the specification and it's better to have it and
never need than not have it and realize we needed it.

Reviewed-by: Lyude <cpaul@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-9-git-send-email-paulo.r.zanoni@intel.com