xfree86: Move secondary GPU autoconfig loop break

Commit d458f84e (xfree86: Fix autoconfig secondary GPU devices)
introduced a potential segmentation fault when all GPU devices were
assigned to a different seat than the one being configured. This commit
fixes that bug by moving the break on NULL before the code which causes
the crash.

The other potential fix for this bug is to leave the break where it is
and add a check for NULL in the if block to ensure that all NULLs end up
in the if case. That fix maintains the invariant that the
conf_screen->screen_gpu_devices list ends with NULL while the one in
this commit does not. Inspection of the code reveals that the code does
not depend on that invariant. Since moving the break is both simpler and
more proper than the other fix, it is the fix chosen for this commit.

Signed-off-by: Jacob Cherry <jcherry@nvidia.com>
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