drm: Remove lock interfaces from GEM VRAM helpers

The lock functions and the locked-pin/unpin functions of GEM VRAM 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/20190613073041.29350-9-tzimmermann@suse.de