drm/vram: Provide vmap and vunmap operations for GEM VRAM objects

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: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911120352.20084-2-tzimmermann@suse.de