i965: Decode three-source register types directly.

i965: Decode three-source register types directly.

Three-source instructions use a different encoding for register types
(and have a much more limited set to choose from).

Previously, we translated those into BRW_REGISTER_TYPE_* values, then
reused the existing reg_encoding mapping.

Doing it directly is more straightforward and actually less code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>