drm/i915/gen9: Give one extra block per line for SKL plane WM calculations

drm/i915/gen9: Give one extra block per line for SKL plane WM calculations

The bspec was updated a couple weeks ago to add an extra block per line
to plane watermark calculations for linear pixel formats.

Bspec update 115327 description:
  "Gen9+ - Updated the plane blocks per line calculation for linear
  cases. Adds +1 for all linear cases to handle the non-block aligned
  stride cases."

Cc: Lyude <cpaul@redhat.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470344880-27394-1-git-send-email-matthew.d.roper@intel.com
Reviewed-by: Lyude <cpaul@redhat.com>