drm/dp: Move DisplayPort helpers into separate helper module

Move DisplayPort functions into a separate module to reduce the size
of the KMS helpers. Select DRM_DP_HELPER for all users of the code. To
avoid naming conflicts, rename drm_dp_helper.c to drm_dp.c

This change can help to reduce the size of the kernel binary. Some
numbers from a x86-64 test build:

Before:
	drm_kms_helper.ko:	447480 bytes

After:
	drm_dp_helper.ko:	216632 bytes
	drm_kms_helper.ko:	239424 bytes

For early-boot graphics, generic DRM drivers, such as simpledrm,
require DRM KMS helpers to be built into the kernel. Generic helper
functions for DisplayPort take up a significant portion of DRM KMS
helper library. These functions are not used by generic drivers and
can be loaded as a module.

v3:
	* fix include statement in DRM selftests
v2:
	* move DP helper code into dp/ (Jani)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220114114535.29157-4-tzimmermann@suse.de