drm/panel: Select DRM_DP_HELPER for DRM_PANEL_EDP

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 <[email protected]>
Fixes: 9d6366e743f3 ("drm: fb_helper: improve CONFIG_FB dependency")
Reported-by: Naresh Kamboju <[email protected]>
Reported-by: Linux Kernel Functional Testing <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Link: https://lore.kernel.org/dri-devel/CA+G9fYvN0NyaVkRQmA1O6rX7H8PPaZrUAD7=RDy33QY9rUU-9g@mail.gmail.com/
Link: https://lore.kernel.org/all/[email protected]/
Cc: Thomas Zimmermann <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Dave Airlie <[email protected]>