drm/vmwgfx: Fix some static checker warnings

Fix some minor issues that Coverity spotted in the code. None
of that are serious but they're all valid concerns so fixing
them makes sense.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609172307.131929-5-zackr@vmware.com