drm/i915/mtl: Re-use ADL-P's "DC off" power well

As with ADL-P, MTL's "DC off" power well should be a dependency of the
PGC and PGD power wells, not the entire PG2 well.  In fact, the DC5/DC6
requirements between the two platforms are the same, so the Xe_LPD "DC
off" well definition can just be re-used for Xe_LPD+.

Bspec: 49193
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230418220446.2205509-3-radhakrishna.sripada@intel.com