drm/vmwgfx: Fix framebuffer modesetting

Must set SVGA_NUM_REG_GUEST_DISPLAY before setting up the display information.

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