drm/gma500: Move fbdev code into separate source file

Move the fbdev emulation from framebuffer.c to fbdev.c. Only build
the source code if the Kconfig symbol has been selected. Remaining in
framebuffer.c is gma500's code for DRM framebuffers. No functional
changes.

v2:
	* remove 'extern' from function declaration (Patrik)
	* declare empty init/fini functions as 'static inline' (kernel
	  test robot)
	* rebase onto vm_flags_set()
	* typo fixes in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313151610.14367-3-tzimmermann@suse.de