The implementation of vmap and vunmap for GEM VRAM helpers is
already in PRIME helpers. The patch moves the operations to separate
functions and exports them for general use.

v3:
	* remove v2's obsolete note on ref-counting
v2:
	* fix documentation
	* add cross references to function documentation
	* document (the lack of) ref-counting for GEM VRAM BO mappings
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Acked-by: NGerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911120352.20084-2-tzimmermann@suse.de