i965: Move MRF register asserts out of brw_reg.h

i965: Move MRF register asserts out of brw_reg.h

In a later patch we will make BRW_MAX_MRF return a different value depending
on the hardware generation, but it is inconvenient to add a gen parameter
to the brw_reg functions only for the assertions, so move these to places where
we have the hardware generation available.

Ken suggested to add the asserts to brw_set_src0 and brw_set_dest since that
would make sure that we catch all uses of MRF registers, even those coming
from modules that generate native code directly, like blorp. Unfortunately,
this is very late in the process which can make things harder to debug, so add
asserts to the generator as well.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>