drm/i915: fix the watermark result selection on glk/gen10+

On these platforms we're supposed to unconditonally pick the method 2
result instead of the minimum.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-4-paulo.r.zanoni@intel.com