drm/cma-helper: Make default object functions the default

As GEM object functions are now mandatory, DRM drivers based on CMA
helpers either set them in their implementation of gem_create_object,
or use the default via drm_gem_cma_create_object_default_funcs().

Simplify this by setting the default CMA object functions for all
objects that don't have any functions of their own. Follows the pattern
of similar code in SHMEM and VRAM helpers. The function
drm_gem_cma_create_object_default_funcs() is redundant and therefore
being removed.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106131632.6796-1-tzimmermann@suse.de