drm/vram: Implement lazy unmapping for GEM VRAM buffers

Frequent mapping and unmapping a buffer object adds overhead for
modifying the page table and creates debug output. Unmapping a buffer
is only required when the memory manager evicts the buffer from its
current location.

v4:
	* WARN_ON if buffer is still mapped during BO cleanup

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reported-and-tested-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906122056.32018-5-tzimmermann@suse.de