drm/i915/power: move dc state members to struct i915_power_domains

There's only one reference to the struct intel_dmc members dc_state,
target_dc_state, and allowed_dc_mask within intel_dmc.c, begging the
question why they are under struct intel_dmc to begin with.

Moreover, the only references to i915->display.dmc outside of
intel_dmc.c are to these members.

They don't belong. Move them from struct intel_dmc to struct
i915_power_domains, which seems like a more suitable place.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301122944.1298929-1-jani.nikula@intel.com