drm/vmwgfx: Fix some static checker warnings

[ Upstream commit 74231041 ]

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


Stable-dep-of: 517621b7 ("drm/vmwgfx: Fix possible null pointer derefence with invalid contexts")
Signed-off-by: Sasha Levin <sashal@kernel.org>