glamor: Only use dual blending with GLSL >= 1.30

It can't be used with older GLSL. Fixes a crash when attempting to
anyway.

Closes: xorg/xserver#97
Fixes: e7308b6c "glamor: Add support for CA rendering in a single pass."
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(Cherry picked from commit 5bfca003)