drm/i915/psr: Avoid any PSR stuff on platforms without support.

We really don't want to setup vfuncs and lock mutexes on
platforms that has no support to PSR.

Also we know what platforms they are so let's do it quietly.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170907230041.22978-2-rodrigo.vivi@intel.com