drm/fb-helper: Move generic fbdev emulation into separate source file

Move the generic fbdev implementation into its own source and header
file. Adapt drivers. No functional changes, but some of the internal
helpers have been renamed to fit into the drm_fbdev_ naming scheme.

v3:
* rename drm_fbdev.{c,h} to drm_fbdev_generic.{c,h}
* rebase onto vmwgfx changes
* rebase onto xlnx changes
* fix include statements in amdgpu

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-22-tzimmermann@suse.de