Merge drm_fb_memcpy() and drm_fb_memcpy_toio() into a drm_fb_memcpy()
that uses struct iosys_map for buffers. The new function also supports
multi-plane color formats. Convert all users of the original helpers.

v2:
	* rebase onto refactored mgag200
	* use drm_formap_info_bpp() (Sam)
	* do static init in hyperv and mgag200 (Sam)
	* update documentation (Sam)
	* add TODO on vaddr location (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808125406.20752-4-tzimmermann@suse.de