drm/vram: Don't export driver callback functions for PRIME

PRIME functionality is now provided by GEM object functions. The driver
callback functions are obsolete. So this patch renames them and turns
them into static internal functions of the VRAM helper library. The
implementation of gem_prime_mmap is now unused and the patch removes it.

v3:
	* kept each renamed function at its original location within file
	* kept documentation

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702115012.4418-6-tzimmermann@suse.de