drm/fb-helper: Export helpers for marking damage areas

Export drm_fb_helper_damage() and drm_fb_helper_damage_range(), which
handle damage areas for fbdev emulation. This is a temporary export
that allows to move the DRM I/O helpers for fbdev into drivers. Only
fbdev-generic and i915 need them. Both will be updated to implement
damage handling by themselves and the exported functions will be removed.

v4:
	* update interfaces

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230530151228.22979-11-tzimmermann@suse.de