drm/i915: Fix maxfifo watermark calc on vlv cursor planes

A typo resulted in the watermarks for cursor planes not being calculated
correctly.  Fixed the typo.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>