drm/{i915, xe}: Implement fbdev emulation as in-kernel client

drm/{i915, xe}: Implement fbdev emulation as in-kernel client

Replace all code that initializes or releases fbdev emulation
throughout the driver. Instead initialize the fbdev client by a
single call to intel_fbdev_setup() after i915 has registered its
DRM device. Just like similar code in other drivers, i915 fbdev
emulation now acts like a regular DRM client. Do the same for xe.

The fbdev client setup consists of the initial preparation and the
hot-plugging of the display. The latter creates the fbdev device
and sets up the fbdev framebuffer. The setup performs display
hot-plugging once. If no display can be detected, DRM probe helpers
re-run the detection on each hotplug event.

A call to drm_client_dev_unregister() releases all in-kernel clients
automatically. No further action is required within i915. If the fbdev
framebuffer has been fully set up, struct fb_ops.fb_destroy implements
the release. For partially initialized emulation, the fbdev client
reverts the initial setup. Do the same for xe and remove its call to
intel_fbdev_fini().

v8:
- setup client in intel_display_driver_register (Jouni)
- mention xe in commit message

v7:
- update xe driver
- reword commit message

v6:
- use 'i915' for i915 device (Jouni)
- remove unnecessary code for non-atomic mode setting (Jouni, Ville)
- fix function name in commit message (Jouni)

v3:
-  as before, silently ignore devices without displays

v2:
-  let drm_client_register() handle initial hotplug
-  fix driver name in error message (Jani)
-  fix non-fbdev build (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409081029.17843-7-tzimmermann@suse.de
Signed-off-by: Jani Nikula <jani.nikula@intel.com>