drm/i915: Simplify condition to keep DMC active during S0ix

For S0ix we want to deinit power domains (and so deactivate the DMC
firmware) exactly when the platform supports the DC9 state. To reach
S0ix we need DC9 on these platforms (for which the DMC FW needs to be
deactivated) while to reach S0ix on the rest of the DMC platforms we
need DC6 (which needs the DMC FW to stay active).

Simplify the condition accordingly so it will be automatically
correct for upcoming DC9 platforms like ICL.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180822112602.27543-1-imre.deak@intel.com