drm/i915: Populate mem_freq in init_gt_powerwave()

init_clock_gating() is too late to read out the mem_freq. We already
want to print out the GPU MHz numbers before it's called. Move the
mem_freq setup to init_gt_powersave().

v2: Also kill the CHV_CZ_CLOCK_FREQ_MODE_* defines

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>