drm/vmwgfx: drop printing the TTM refcount for debugging

That is something TTM internal which is about to get dropped.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ian Forbes <ian.forbes@broadcom.com>
Link: https://lore.kernel.org/r/20250616130726.22863-1-christian.koenig@amd.com