i965/fs: Reset reg_offset of the original destination to zero in compute_to_mrf().

Prevents an assertion failure in the following commit.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

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

 	       /* Found the creator of our MRF's source value. */
 	       scan_inst->dst.file = MRF;
                scan_inst->dst.nr = inst->dst.nr;
+               scan_inst->dst.reg_offset = 0;
 	       scan_inst->saturate |= inst->saturate;
 	       inst->remove(block);
 	       progress = true;