drm/panel: Fix build error when CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20=y && CONFIG_DRM_DISPLAY_HELPER=m

If CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20=y && CONFIG_DRM_DISPLAY_HELPER=m,
bulding fails:

drivers/gpu/drm/panel/panel-samsung-atna33xc20.o: In function `atana33xc20_probe':
panel-samsung-atna33xc20.c:(.text+0x744): undefined reference to
 `drm_panel_dp_aux_backlight'
make: *** [vmlinux] Error 1

Let CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20 select DRM_DISPLAY_DP_HELPER and
CONFIG_DRM_DISPLAY_HELPER to fix this error.

Fixes: 32ce3b32 ("drm/panel: atna33xc20: Introduce the Samsung ATNA33XC20 panel")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Gao Chao <gaochao49@huawei.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220524024551.539-1-gaochao49@huawei.com