i965: Implement fs backend for ARB_sample_shading

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
i965: Enable the features required for arb_sample_shading on gen6

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
i965: Enable the features required for arb_sample_shading on gen7

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
i965: Enable ARB_sample_shading on intel hardware >= gen6

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
i965: Fix FS backend