mesa: don't compile integer clear shaders for unsupported APIs

mesa: don't compile integer clear shaders for unsupported APIs

Discovered while running the Khronos conformance test suite and
receiving "implementation error: meta program compile failed."

This bug was recently introduced by the i965 clear patch set and would
only be detected while using the ES2 API and only on gen6+ hardware.

Signed-off-by: Oliver McFadden <oliver.mcfadden@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>