drm/i915/bxt: Expose DC5 entry count

For bxt CSR firmware exposes a count of dc5 entries. Expose
it through debugs

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>