u_format: Add missing BE swizzles for R8SG8SB8UX8U_NORM

Compare to the swizzles for PIPE_FORMAT_R10G10B10A2_UNORM and
R5SG5SB6U_NORM.  This should fix our last BE pack/unpack failure.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <!7776>
127 jobs for !7776 with format_tests_be in 19 minutes and 32 seconds (queued for 6 seconds)
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