drm/i915/skl: Updated the gen9_enable_rps function

drm/i915/skl: Updated the gen9_enable_rps function

On SKL, GT frequency is programmed in units of 16.66 MHZ units compared
to 50 MHZ for older platforms. Also the time value specified for Up/Down EI &
Up/Down thresholds are expressed in units of 1.33 us, compared to 1.28
us for older platforms. So updated the gen9_enable_rps function as per that.

v2: Updated to use new macro GT_INTERVAL_FROM_US

v3: Removed the initial setup of certain registers, from gen9_enable_rps,
    which gets overridden later from gen6_set_rps (Damien)

v4: Removed the enabling of rps interrupts, from gen9_enable_rps.
    To be done from intel_gen6_powersave_work only, as done for other
    platforms also.

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>