i965/emit: Do the sampler index adjustment directly in header.0.3

i965/emit: Do the sampler index adjustment directly in header.0.3

Prior to this commit, the adjust_sampler_state_pointer function took an
extra register that it could use as scratch space.  The usual candidate was
the destination of the sampler instruction.  However, if that register ever
aliased anything important such as the sampler index, this would scratch
all over important data.  Fortunately, the calculation is such that we can
just do it in place and we don't need the scratch space at all.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>