drm/i915: Add gt_act_freq_mhz sysfs file

Currently the 'gt_cur_freq_mhz' file shows the actual GPU frequency on
VLV/CHV, and the last requested frequency on other platforms. Change the
meaning of the file on VLV/CHV to follow the the other platforms, and
introduce a new file 'gt_act_freq_mhz' which shows the actual frequency
on all platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>