drm/atmel-hlcdc/atmel_hlcdc_plane: Fix documentation formatting and add missing description

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c:283:6: warning: no previous prototype for ‘atmel_hlcdc_plane_setup_scaler’ [-Wmissing-prototypes]
 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c:44: warning: cannot understand function prototype: 'struct atmel_hlcdc_plane_state '

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112190039.2785914-18-lee.jones@linaro.org

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c

@@ -19,7 +19,7 @@
 #include "atmel_hlcdc_dc.h"
 
 /**
- * Atmel HLCDC Plane state structure.
+ * struct atmel_hlcdc_plane_state - Atmel HLCDC Plane state structure.
  *
  * @base: DRM plane state
  * @crtc_x: x position of the plane relative to the CRTC
@@ -34,6 +34,7 @@
  * @disc_y: y discard position
  * @disc_w: discard width
  * @disc_h: discard height
+ * @ahb_id: AHB identification number
  * @bpp: bytes per pixel deduced from pixel_format
  * @offsets: offsets to apply to the GEM buffers
  * @xstride: value to add to the pixel pointer between each line