i965: Add missing sample shading bits to Gen8's 3DSTATE_PS_EXTRA.

i965: Add missing sample shading bits to Gen8's 3DSTATE_PS_EXTRA.

v2: Also set the "oMask Present to Render Target" bit, which is required
    for shaders that write oMask.  Otherwise the hardware won't expect
    the extra data.

v3: Add missing _NEW_MULTISAMPLE (caught by Eric).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>