[POWERPC] XilinxFB: rename failout labels to reflect failure

Labels and gotos are used in xilinxfb_assign to unwind allocations
on device registration failures.  Rename the labels to reflect the
error which occured.  This change is being made to make it easier
to add new failout paths (which occurs in a subsuquent patch) and
to make reviewing the failout path easier.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andrei Konovalov <akonovalov@ru.mvista.com>