i965: Remove duplicate MRF writes in the FS backend.

i965: Remove duplicate MRF writes in the FS backend.

This is quite common for multitexture sampling, and not only cuts down
on the second and later set of MOVs, but typically also allows
compute-to-MRF on the first set.

No statistically siginficant performance difference in nexuiz (n=3),
but it reduces instruction count in one of its shaders and seems like
a good idea.