i965/vs: Expose the payload registers to the register allocator.

i965/vs: Expose the payload registers to the register allocator.

For now, nothing else can get allocated over them.  That may change at
some point in the future.

This also means that base_reg_count can be computed without knowing the
number of registers used for the payload, which is required if we want
to allocate the register set once at context creation time.

See commit 551e1cd44f6857f7e29ea4c8f892da5a97844377, which implemented
virtually identical code in the FS backend.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>