drm/i915: Clean up L3 SQC register field definitions

No need for hard-coding the register value, the corresponding fields are
defined properly in BSpec.

No functional change.

v2:
- Rebased on BXT L3 SQC tuning patch merged meanwhile.

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1462280061-1457-3-git-send-email-imre.deak@intel.com