drm/i915: Implement display w/a 1140 for glk/cnl

Display w/a #1140 tells us we have to program the transition
watermark to the minimum value on glk/cnl. Let's do that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228203552.30273-4-ville.syrjala@linux.intel.com