drm/vmwgfx: Fix fence event code

The commit "vmwgfx: Rework fence event action" introduced a number of bugs
that are fixed with this commit:

a) A forgotten return stateemnt.
b) An if statement with identical branches.

Cc: <stable@vger.kernel.org>
Reported-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>