drm/i915: W/A for underruns with WM1+ disabled on icl

drm/i915: W/A for underruns with WM1+ disabled on icl

Disabling WM1+ on ICL causes tons of underruns with
linear/X-tiled framebuffers. We can avoid this by flipping
on a chicken bit affecting the way the hw fill the FIFO.
This may not be the final solution but should hopefully
avoid some underruns in the meantime.

v2: Apparently PIPE_CHICKEN is icl+ only

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190204202232.27153-1-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>