drm: Rename dp/ to display/

Rename dp/ to display/ to account for additional display-related
helpers, such as HDMI. Update all related include statements. No
functional changes.

Various drivers, such as i915 and amdgpu, use similar naming scheme
by putting code for video-output standards into a local display/
directory. The new directory's name is aligned with this convention.

v2:
	* update commit message (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421073108.19226-3-tzimmermann@suse.de