i965: Set fs_inst::base_mrf = -1 by default.

On MRF platforms, we need to set base_mrf to the first MRF value we'd
like to use for the message.  On send-from-GRF platforms, we set it to
-1 to indicate that the operation doesn't use MRFs.

As MRF platforms are becoming increasingly a thing of the past, we've
forgotten to bother with this.  It makes more sense to set it to -1 by
default, so we don't have to think about it for new code.

I searched the code for every instance of 'mlen =' in brw_fs*cpp, and
it appears that all MRF-based messages correctly program a base_mrf.

Forgetting to set base_mrf = -1 can confuse the register allocator,
causing it to think we have a large fake-MRF region.  This ends up
moving the send-with-EOT registers earlier, sometimes even out of
the g112-g127 range, which is illegal.  For example, this fixes
illegal sends in Piglit's arb_gpu_shader_fp64-layout-std430-fp64-shader,
which had SSBO messages with mlen > 0 but base_mrf == 0.

Cc: [email protected]
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Francisco Jerez <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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