drm/vmwgfx: Fix gcc-7.1.1 warning

The current code does not look correct, and the reason for it is
probably lost.  Since this now generates a compiler warning,
fix it to what makes sense.

Cc: <stable@vger.kernel.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>