drm/display: Introduce a DRM display-helper module

Replace the DP-helper module with a display-helper module. The
support for DisplayPort becomes an internal option that drivers
have to select. Update all related Kconfig and Makefile rules.

Besides the existing code for DisplayPort, the new module will
contain helpers for other video-output standards, such as HDMI.
Drivers will have to select their required video-output helpers.

Linking all display-related code into a single module avoids the
proliferation of small kernel modules.

The module parameters drm_dp_cec_unregister_delay, dp_aux_i2c_speed_khz,
and dp_aux_i2c_transfer_size are moving from the drm_dp_helper namespace
to drm_display_helper.

v2:
	* mention module parameters in commit message (Javier)
	* distiguish between display module and DP support in Kconfig
	* update Makefile rules for DP helpers
	* move Kconfig rules into separate file under display/

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421073108.19226-4-tzimmermann@suse.de