drm/format-helper: Rename parameter vmap to src

Rename the parameter vmap to src in all functions. The parameter
contains the locations of the source data and the new name says that.

v3:
	* fix typo in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808125406.20752-15-tzimmermann@suse.de