drm/i915: Be defensive and don't assume PSR has any commit to sync against

If the previous modeset commit has completed and is no longer part of
the crtc state, skip waiting for it.

Ville pointed out that, in fact, the commit is never removed after a
modeset so the only way we could see a NULL here should be if there was
never a commit attached. Nevertheless, we have the evidence it can be
NULL and it has been defended against elsewhere, for example commit
93313538 ("drm/i915: Pass idle crtc_state to intel_dp_sink_crc").

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107792


Fixes: c44301fc ("drm/i915: Allow control of PSR at runtime through debugfs, v6")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904162902.2578-1-chris@chris-wilson.co.uk