i965: Fix register types in dump_instructions().

i965: Fix register types in dump_instructions().

This regressed when I converted BRW_REGISTER_TYPE_* to be an abstract
type that doesn't match the hardware description.  dump_instruction()
was using reg_encoding[] from brw_disasm.c, which no longer matches
(and was incorrect for Gen8+ anyway).

This patch introduces a new function to convert the abstract enum values
into the letter suffix we expect.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reported-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>