i965/fs: Handle MRF destinations in lower_integer_multiplication().

i965/fs: Handle MRF destinations in lower_integer_multiplication().

The lowered code reads from the destination, which isn't possible from
message registers.

Fixes the following dEQP tests on SNB:

    dEQP-GLES3.functional.shaders.precision.int.highp_mul_fragment
    dEQP-GLES3.functional.shaders.precision.int.mediump_mul_fragment
    dEQP-GLES3.functional.shaders.precision.int.lowp_mul_fragment

Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Tested-by:  Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>