drm/i915: Do not warn on PG2 enabled in gen9_disable_dc5()

PG2 enabled is not a requirement for disabling DC5. It's just one
of the reasons why the DMC wouldn't enter DC5. During modeset we don't
care about PG2 from a DC perspective, only the fact that DC5/DC6 is not
allowed.

Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447084107-8521-9-git-send-email-patrik.jakobsson@linux.intel.com