drm/i915: Generalize the PPS vlv_pipe_check() stuff

Restate the vlv_pipe_check() stuff in terms of PPS index
(rather than pipe, which it is on VLV/CHV) so that we can
reuse this same mechanim on other platforms as well.

Cc: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221125173156.31689-4-ville.syrjala@linux.intel.com