drm/display: Select DP helper for DRM_DP_AUX_CHARDEV and DRM_DP_CEC

The DRM_DP_AUX_CHARDEV and DRM_DP_CEC Kconfig symbols enable code that use
DP helper functions, that are only present if CONFIG_DRM_DISPLAY_DP_HELPER
is also enabled.

But these don't select the DRM_DISPLAY_DP_HELPER symbol, meaning that it
is possible to enable any of them without CONFIG_DRM_DISPLAY_DP_HELPER.

That will lead to the following linking errors with the mentioned config:

  LD      vmlinux.o
  MODPOST vmlinux.symvers
  MODINFO modules.builtin.modinfo
  GEN     modules.builtin
  LD      .tmp_vmlinux.kallsyms1
  KSYMS   .tmp_vmlinux.kallsyms1.S
  AS      .tmp_vmlinux.kallsyms1.S
  LD      .tmp_vmlinux.kallsyms2
  KSYMS   .tmp_vmlinux.kallsyms2.S
  AS      .tmp_vmlinux.kallsyms2.S
  LD      vmlinux
  SYSMAP  System.map
  SORTTAB vmlinux
  OBJCOPY arch/arm64/boot/Image
  MODPOST modules-only.symvers
ERROR: modpost: "drm_dp_dpcd_write" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
ERROR: modpost: "drm_dp_read_desc" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
ERROR: modpost: "drm_dp_dpcd_read" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:134: modules-only.symvers] Error 1
make[1]: *** Deleting file 'modules-only.symvers'
make: *** [Makefile:1749: modules] Error 2

Besides making these symbols to select CONFIG_DRM_DISPLAY_DP_HELPER, make
them to depend on DRM_DISPLAY_HELPER, since can't be enabled without it.

Note: It seems this has been an issue for a long time but was made easier
to reproduce after the commit 1e0f6642 ("drm/display: Introduce a DRM
display-helper module"). Adding a Fixes: tag just to make sure that this
fix will be picked for stable once the mentioned change also lands there.

Fixes: 1e0f6642 ("drm/display: Introduce a DRM display-helper module")
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220428082244.390859-1-javierm@redhat.com

drivers/gpu/drm/display/Kconfig

@@ -31,7 +31,8 @@ config DRM_DISPLAY_HDMI_HELPER
 
 config DRM_DP_AUX_CHARDEV
 	bool "DRM DP AUX Interface"
-	depends on DRM
+	depends on DRM && DRM_DISPLAY_HELPER
+	select DRM_DISPLAY_DP_HELPER
 	help
 	  Choose this option to enable a /dev/drm_dp_auxN node that allows to
 	  read and write values to arbitrary DPCD registers on the DP aux
@@ -39,7 +40,8 @@ config DRM_DP_AUX_CHARDEV
 
 config DRM_DP_CEC
 	bool "Enable DisplayPort CEC-Tunneling-over-AUX HDMI support"
-	depends on DRM
+	depends on DRM && DRM_DISPLAY_HELPER
+	select DRM_DISPLAY_DP_HELPER
 	select CEC_CORE
 	help
 	  Choose this option if you want to enable HDMI CEC support for