drm/i915: Fix kerneldoc for i915 atomic plane code

Description of the 'state' parameter for intel_plane_destroy_state() was
missing and the intel_atomic_plane.c file section heading did not match
drm.tmpl.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>