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.