drm/i915/vlv: Workaround a punit issue in DDR data rate for 1333.

For DDR data rate reporting by Punit in PUNIT_GPU_FREQ_STS, the actual
data encoding is 00b=800, 01b=1066, 10b=1333, 11b=1333.

Some premium VLV sku will get the DDR_DATA_RATE set as 11.  As a result,
the turbo frequency reporting will be incorrect without this workaround.

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>