drm/format-helper: Convert drm_fb_swab() to struct iosys_map

Convert drm_fb_swab() to use struct iosys_map() and convert users. The
new interface supports multi-plane color formats, but implementation
only supports a single plane for now.

v2:
	* use drm_format_info_bpp() (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/20220808125406.20752-5-tzimmermann@suse.de