i965: step message register allocation
i965: step message register allocation

The system routine requires m0 be reserved for saving off architectural
state. Moved the allocation to start at 2 instead of 0.

Signed-off-by: Ben Widawsky <73675debcd8a436be48ec22211dcf44fe0df0a64@bwidawsk.net>
Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

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