mesa: Add dispatch API stubs for ARB_compute_shader.
fixup! mesa: Implement MAX_COMPUTE_WORK_GROUP_SIZE constant.
mesa: Implement MAX_COMPUTE_WORK_GROUP_INVOCATIONS constant.
i965/gen7: Allow ARB_compute_shader to be enabled via env var.

This will allow testing of compute shader functionality before it is
completed.
fixup! mesa: Implement MAX_COMPUTE_WORK_GROUP_INVOCATIONS constant.