Intel i915 support for RC6

[Jan Kokemüller]

 > Is '7' reasonable and, if so, what do the other two bits do?

The value of the sysctl is interpreted as a bitmask. There are three 
different RC6 levels: RC6, RC6p and RC6pp. RC6pp is the deepest one with 
the largest power savings.
Setting it to 7 enables all modes. dmesg then says:

"info: [drm] Enabling RC6 states: RC6 on, RC6p on, RC6pp on"

I would set it to 7 unless it makes the system unstable. RC6p seems to 
be broken on Sandy Bridge but it is automatically disabled there.