drm/gem: Move drm_gem_fb_prepare_fb() to GEM atomic helpers

The function drm_gem_fb_prepare_fb() is a helper for atomic modesetting,
but currently located next to framebuffer helpers. Move it to GEM atomic
helpers, rename it slightly and adopt the drivers. Same for the rsp
simple-pipe helper.

Compile-tested with x86-64, aarch64 and arm. The patch is fairly large,
but there are no functional changes.

v3:
	* remove out-comented line in drm_gem_framebuffer_helper.h
	  (Maxime)
v2:
	* rename to drm_gem_plane_helper_prepare_fb() (Daniel)
	* add tutorial-style documentation

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210222141756.7864-1-tzimmermann@suse.de