drm: Fix kerneldoc in drm_plane_helper.c

Ville ocd'ed the parameter name, but forgot to update the docs!

Fixes: df86af9133b4 ("drm/plane-helper: Add drm_plane_helper_check_state()")
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-21-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_plane_helper.c

@@ -199,7 +199,7 @@ EXPORT_SYMBOL(drm_plane_helper_check_state);
  * @crtc: owning CRTC of owning plane
  * @fb: framebuffer to flip onto plane
  * @src: source coordinates in 16.16 fixed point
- * @dest: integer destination coordinates
+ * @dst: integer destination coordinates
  * @clip: integer clipping coordinates
  * @rotation: plane rotation
  * @min_scale: minimum @src:@dest scaling factor in 16.16 fixed point