Don't override the GEM object functions unconditionally. If the driver
sets the GEM functions, VRAM helpers will not set them. The idea has been
taken from SHMEM helpers.

v2:
	* updated the commit message (Sam)
	* document the new feature for drm_gem_vram_create()
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200922145700.25413-3-tzimmermann@suse.de