video: Provide constants for VGA I/O range

Provide VGA_FB_ constants for the VGA framebuffer I/O range and convert
fbdev code. In the case of vga16fb, this is a rename of the existing
constants VGA_FB_PHYS and VGA_FB_PHYS_LEN.

v2:
	* clarify relationship with old constants in vga16fb (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220718072322.8927-9-tzimmermann@suse.de