drm/i915/psr: Avoid DPCD reads when panel does not support PSR

Ville noticed that we are unncessarily reading DPCD's after knowing
panel did not support PSR. Looks like this check that was present
earlier got removed unintentionally, let's put it back.

While we do this, add the PSR version number in the debug print.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Tarun Vyas <tarun.vyas@intel.com>
Reviewed-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-1-dhinakaran.pandiyan@intel.com