drm/i915: Setup static bias for GPU

Based on the spec, Setting up static BIAS for GPU to improve the
rps performace.

v2: rename reg defn to match spec. (Ville)

v3: Updated bias setting for chv (Deepak)

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