drm/cma-helper: Rename symbols from drm_cma_gem_ to drm_gem_cma_

This fixes the naming of several symbols within CMA helpers. No functional
changes are made.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605073247.4057-2-tzimmermann@suse.de