drm/gma500: fbdev: Use a DRM client buffer

drm/gma500: fbdev: Use a DRM client buffer

Replace the internal DRM framebuffer with a DRM client buffer. The
client buffer allocates the DRM framebuffer on a file and also uses
GEM object handles via the regular ADDFB2 interfaces. This unifies
framebuffer allocation for clients in user space and gma500's internal
fbdev emulation.

Also simplify the clean-up side of the fbdev emulation.

Later patches will allow for streamlining gma500's framebuffer code and
DRM's fbdev emulation in general.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patch.msgid.link/20260206133458.226467-5-tzimmermann@suse.de