fbdev/vga16fb: Create EGA/VGA devices in sysfb code

fbdev/vga16fb: Create EGA/VGA devices in sysfb code

Move the device-creation from vga16fb to sysfb code. The driver's
videomode checks are independent from device creation, so move them
into vga16fb's probe function. This will allow to create the module
init/exit code automatically.

The vga16fb driver requires a screen_info for type VIDEO_TYPE_VGAC
or VIDEO_TYPE_EGAC. Such code is nowhere present in the kernel, except
for some MIPS systems. It's not clear if the vga16fb driver actually
works in practice.

v2:
	* keep driver name to "vga16fb" (Javier)
	* give rational for moving mode checks (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220718072322.8927-3-tzimmermann@suse.de