drm/i915/psr: Remove PSR2 TODO error handling

We are already handling all PSR2 errors, so we can drop this TODO.

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003205031.32474-3-jose.souza@intel.com