drm/i915/psr: Sprinkle cpu_transcoder variables around

Reduce the 'intel_dp' stuff a bit by introducing local
cpu_transcoder variables.

Ideally I'd like the whole PSR code to stop using intel_dp
except during a full modeset, but dunno yet if that's
possible. But the less 'intel_dp' we have sprad around
the easier that should be to figure out eventually.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230411191429.29895-9-ville.syrjala@linux.intel.comReviewed-by: Jouni Högander <jouni.hogander@intel.com>