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: NThomas Daniel <thomas.daniel@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>