i965/fs: Allow scalar source regions on SNB math instructions.

i965/fs: Allow scalar source regions on SNB math instructions.

I haven't found any evidence that this isn't supported by the
hardware, in fact according to the SNB hardware spec:

 "The supported regioning modes for math instructions are align16,
  align1 with the following restrictions:
   - Scalar source is supported.
  [...]
   - Source and destination offset must be the same, except the case of
     scalar source."

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>