nv50/ir: improve instruction pipelining on gm107

nv50/ir: improve instruction pipelining on gm107

This makes use of scheduling control codes which are very useful
for improving the instruction pipelining.

This patch will increase performance on Maxwell GPUs by, at least,
x1.5 up to x3.5 for some benchmarks.

Although this has been fairly well tested, I would not be suprised
if someone hit a corner case somewhere. That way, the scheduler
is enabled by default but it can be deactivated by using
NV50_PROG_SCHED=0.

Thanks to Scott Gray for the reverse engineering work available from
https://github.com/NervanaSystems/maxas/wiki/Control-Codes.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Pierre Moreau <pierre.morrow@free.fr>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Jan Vesely <jan.vesely@rutgers.edu>