i965: Gen4-5: Include alpha func/ref in program key

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Gen4-5: Setup discard masks for MRT alpha test

The same setup is required here as when the user-provided shader
explicitly uses KIL or discard.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Gen4-5: Implement alpha test in shader for MRT

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>