drm/cma-helper: Pass GEM CMA object in public interfaces

Change all GEM CMA object functions that receive a GEM object
of type struct drm_gem_object to expect an object of type
struct drm_gem_cma_object instead.

This change reduces the number of upcasts from struct drm_gem_object
by moving them into callers. The C compiler can now verify that the
GEM CMA functions are called with the correct type.

For consistency, the patch also renames drm_gem_cma_free_object to
drm_gem_cma_free. It further updates documentation for a number of
functions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211115120148.21766-4-tzimmermann@suse.de