drm/i915/skl: Updated intel_gpu_freq() and intel_freq_opcode()

On SKL, frequency is specified in units of 16.66 MHZ.
Updated the intel_gpu_freq() and intel_freq_opecode() functions
to do the conversion appropriately.

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>