plane_state can't be NULL anywhere in this function, so the NULL check
at the end is redundant, remove it.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: NImre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1493214013-15580-9-git-send-email-imre.deak@intel.comReviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>