drm/i915/tgl: Move transcoders to pipes' powerwells

When trying to read registers from transcoder C and D while PG3 is ON it
causes unclaimed access warnings. Adding the powerwells for the pipes
fixes the issue, but doesn't match the spec.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817093902.2171-4-lucas.demarchi@intel.com