drm/i915/tgl: Allow DC5/DC6 entry while PG2 is active

On gen12, we no longer need to disable DC5/DC6 when when PG2 is in use
(which translates to cases where we're using VDSC on pipe A).

Bspec: 49193
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200220231843.3127468-1-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>