drm/i915: Remove old PSR2 FIXME about frontbuffer tracking

Our frontbuffer tracking improved over the years + the WA #0884
helped us keep PSR2 enabled while triggering screen updates when
necessary so this FIXME is not valid anymore.

Acked-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181204003403.23361-6-jose.souza@intel.com