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 <[email protected]>
Cc: [email protected]
Signed-off-by: Matt Roper <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Lyude <[email protected]>