drm/atmel-hlcdc/atmel_hlcdc_plane: Staticise local function 'atmel_hlcdc_plane_setup_scaler()'

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]

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/20201112190039.2785914-17-lee.jones@linaro.org