drm: Add client-agnostic setup helper

DRM may support multiple in-kernel clients that run as soon as a DRM
driver has been registered. To select the client(s) in a single place,
introduce drm_client_setup().

Drivers that call the new helper automatically instantiate the kernel's
configured default clients. Only fbdev emulation is currently supported.
Later versions can add support for DRM-based logging, a boot logo or even
a console.

Some drivers handle the color mode for clients internally. Provide the
helper drm_client_setup_with_color_mode() for them.

Using the new interface requires the driver to select
DRM_CLIENT_SELECTION in its Kconfig. For now this only enables the
client-setup helpers if the fbdev client has been configured by the
user. A future patchset will further modularize client support and
rework DRM_CLIENT_SELECTION to select the correct dependencies for
all its clients.

v5:
- add CONFIG_DRM_CLIENT_SELECTION und DRM_CLIENT_SETUP
v4:
- fix docs for drm_client_setup_with_fourcc() (Geert)
v3:
- fix build error
v2:
- add drm_client_setup_with_fourcc() (Laurent)
- push default-format handling into actual clients

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-5-tzimmermann@suse.de

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