drm/fbdev: Select fbdev I/O helpers from modules that require them

Fbdev emulation for SHMEM and TTM requires helpers from the fbdev
subsystem. Select them from the modules that use them instead of the
core DRM module.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241014085740.582287-5-tzimmermann@suse.de