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: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -57,6 +57,7 @@ fs_inst::init(enum opcode opcode, uint8_t exec_size, const fs_reg &dst,
    this->dst = dst;
    this->sources = sources;
    this->exec_size = exec_size;
+   this->base_mrf = -1;
 
    assert(dst.file != IMM && dst.file != UNIFORM);