drm/i915/glk: Fix DMC/DC state idleness calculation

According to BSpec GLK like BXT needs to ignore the idle state of cores
before starting the DMC firmware's DC state handler.

Fixes: dbb28b5c ("drm/i915/DMC/GLK: Load DMC on GLK")
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171003095159.711-2-imre.deak@intel.com