i965: Add support for AVG instruction.

i965: Add support for AVG instruction.

From the Ivy Bridge PRM, Vol4 Part3 p152:

    "The avg instruction performs component-wise integer average of
    src0 and src1 and stores the results in dst. An integer average
    uses integer upward rounding. It is equivalent to increment one to
    the addition of src0 and src1 and then apply an arithmetic right
    shift to this intermediate value."

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>