i965: Mark brw_reg_type and register_file enums as PACKED.

The C99 spec says the type of an enum is implementation defined (but can
be char, signed int, or unsigned int). gcc appears to always give enums
four bytes, even when they can fit in less. It does so because this is
what other compilers seem to do [0] and therefore to maintain ABI
compatibility with them.

gcc has an -fshort-enum flag that tells the compiler to use only as much
space as needed for an enum. Adding __attribute__((__packed__)) to an
enum definition has the same behavior, but on a per-enum basis.

brw_reg_type and register_file are not part of the ABI, so we can safely
mark them as PACKED so that they'll take only a byte, rather than four.

[0] http://gcc.gnu.org/onlinedocs/gcc/Non-bugs.html#index-fshort-enums-3868

Acked-by: Kenneth Graunke <[email protected]>

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