drm/vmwgfx: Delete mmaping functions

No need for stubs, dma-buf.c takes care of that.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-11-daniel.vetter@ffwll.ch