drm/i915/display: Only keep PSR enabled if there is active planes

PSR always had a requirement to only be enabled if there is active
planes but not following that never caused any issues.
But that changes in Alderlake-P, leaving PSR enabled without
active planes causes transcoder/port underruns.

Similar behavior was fixed during the pipe disable sequence by
commit 84030adb

 ("drm/i915/display: Disable audio, DRRS and PSR before planes").

intel_dp_compute_psr_vsc_sdp() had to move from
intel_psr_enable_locked() to intel_psr_compute_config() because we
need to be able to disable/enable PSR from atomic states without
connector and encoder state.

Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210922215242.66683-3-jose.souza@intel.com