drm/panel: Select DRM_DP_HELPER for DRM_PANEL_EDP

As reported in [1], DRM_PANEL_EDP depends on DRM_DP_HELPER. Select
the option to fix the build failure. The error message is shown
below.

  arm-linux-gnueabihf-ld: drivers/gpu/drm/panel/panel-edp.o: in function
    `panel_edp_probe': panel-edp.c:(.text+0xb74): undefined reference to
    `drm_panel_dp_aux_backlight'
  make[1]: *** [/builds/linux/Makefile:1222: vmlinux] Error 1

The issue has been reported before, when DisplayPort helpers were
hidden behind the option CONFIG_DRM_KMS_HELPER. [2]

v2:
	* fix and expand commit description (Arnd)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 9d6366e7 ("drm: fb_helper: improve CONFIG_FB dependency")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/dri-devel/CA+G9fYvN0NyaVkRQmA1O6rX7H8PPaZrUAD7=RDy33QY9rUU-9g@mail.gmail.com/ # [1]
Link: https://lore.kernel.org/all/20211117062704.14671-1-rdunlap@infradead.org/ # [2]
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220203093922.20754-1-tzimmermann@suse.deSigned-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/panel/Kconfig

@@ -106,6 +106,7 @@ config DRM_PANEL_EDP
 	depends on PM
 	select VIDEOMODE_HELPERS
 	select DRM_DP_AUX_BUS
+	select DRM_DP_HELPER
 	help
 	  DRM panel driver for dumb eDP panels that need at most a regulator and
 	  a GPIO to be powered up. Optionally a backlight can be attached so