drm/fbdev-generic: Implement dedicated fbdev I/O helpers

Implement dedicated fbdev helpers for framebuffer I/O instead
of using DRM's helpers. Use an fbdev generator macro for
deferred I/O to create the callbacks. Fbdev-generic was the
only caller of the DRM helpers, so remove them from the helper
module.

v4:
	* generate deferred-I/O helpers
	* use initializer macros for fb_ops
v2:
	* use FB_SYS_HELPERS_DEFERRED option

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230530151228.22979-13-tzimmermann@suse.de