drm/vmwgfx: Reserve first part of VRAM for framebuffer.

The host may be touching this part of VRAM at modesetting,
even if we never use it ourselves, since we blit screen updates from 3D
surfaces. Make sure no DMA buffers are placed in this part of VRAM.

V2: Fix an error check in vmw_surface_dmabuf_pin().

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>