drm/i915: Improve PPS debugs

Always include both the encoder and PPS instance information
in the debug prints so that we know what piece of hardware
we're actually dealing with.

v2: Make sure pps is selected before debug prints/etc. in
    intel_pps_vdd_on_unlocked() on vlv/chv
    There is no pps on pipe C on chv
v3: Allow PPS=INVALID_PIPE for vlv/chv

Cc: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221127155239.26973-1-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>