drm/cma-helper: Remove prime infix from GEM object functions

These functions are not directly related to PRIME interfaces any
longer, but are now GEM object functions. Rename them accordingly
and fix the docs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201123115646.11004-2-tzimmermann@suse.de