drm: Remove functions with kmap-object argument from GEM VRAM helpers

The GEM VRAM functions with kmap-object argument are not required any
longer. Remove them.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-10-tzimmermann@suse.de