r600g: use old shader disassembler by default

r600g: use old shader disassembler by default

New disassembler is not completely isolated yet from further processing
in r600g/sb that is not required for printing the dump, so it has higher
probability to fail in case of any unexpected features in the bytecode.

This patch adds "sbdisasm" flag for R600_DEBUG that allows to use new
disassembler in r600g/sb for shader dumps when shader optimization
is not enabled.

If shader optimization is enabled, new disassembler is used by default.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>