drm/fbdev-dma: Implement fbdev emulation for GEM DMA helpers

Implement fbdev emulation that is optimized for drivers that use
DMA helpers. The buffers may no tbe moveable, may not require damage
handling and have to be located in system memory. This allows fbdev
emulation to operate directly on the buffer and mmap it to userspace.

Besides those constraints, the emulation works like in the generic
code. As an internal DRM client provides, it receives hotplug, restore
and unregister events. The DRM client is independent from the fbdev
probing, which runs on the first successful hotplug event.

The emulation is part of the DMA helper module and not build unless
DMA helpers and fbdev emulation has been configured.

Tested with vc4.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313155138.20584-2-tzimmermann@suse.de