drm/vmwgfx: consolidate ttm object creation and populate

These two functions has the same code in them, create a common
helper function instead.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-2-airlied@gmail.com