drm/nv50/gr: enable ctxprog xfer only when we need it to save power

This patch adds instructions to ctxprog and by doing, impacts context
switching performance.  My testcase showed a 1% performance cost using
glxgears that is a context-switch bound application.

Please test and report bugs/performance/power/other.

Many thanks to Maxim Levitsky for his dedicated work on lowering power
consumption with nouveau.

More patches are coming thanks to his work:

https://bugs.freedesktop.org/show_bug.cgi?id=37922



Signed-off-by: Martin Peres <martin.peres@ensi-bourges.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>