drm/fbdev-emulation: Remove support for legacy emulation

drm/fbdev-emulation: Remove support for legacy emulation

Remove the internal DRM client from fbdev emulation. This has been
required when some DRM drivers provided their own fbdev emulation.

This is no longer the case with commit b55f3bbab891 ("drm/{i915, xe}:
Implement fbdev emulation as in-kernel client") from 2024. Now there's
only a single DRM client for fbdev-emulation that fills out the client
callback functions as required.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patch.msgid.link/20260205144056.416759-1-tzimmermann@suse.de