Prior to SKL, the time period programmed in Up/Down EI & Up/Down
threshold registers was in units of 1.28 micro seconds. But for
SKL, the units have changed (1.333 micro seconds).
Have generalized the implementation of gen6_set_rps_thresholds function,
by removing the hard coding done in it as per 1.28 micro seconds.

v2: Renamed the local variables & removed superfluous comments (Chris)
Signed-off-by: NAkash Goel <akash.goel@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>