drm/vmwgfx: fix update of display surface when resolution changes

The assignment of metadata overwrote the new display resolution values,
hence we'd miss the size actually changed and wouldn't redefine the
surface. This would then lead to command buffer error when trying to
update the screen target (due to the size mismatch), and result in a
VM with black screen.

Fixes: 504901db

 ("drm/vmwgfx: Refactor surface_define to use vmw_surface_metadata")
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>
Cc: stable@vger.kernel.org