drm/cma-helper: Move driver and file ops to the end of header

Restructure the header file for CMA helpers by moving declarations
for driver and file operations to the end of the file. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211115120148.21766-2-tzimmermann@suse.de