R600/SI: Use source scheduler

This is certainly not the last word on scheduling for this target, but
right now this allows a few apps to run / finish with radeonsi, most
notably UT2004 / Lightsmark. They fail to compile some shaders with the
default scheduler because it ends up trying to spill registers, which
we don't support yet (and which is probably a bad idea in general for
performance if it can be avoided).

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>