drm/i915/psr: Update PSR2 SU corruption workaround comment

Turn out it is not a DMC bug it is actually a HW one, so this
workaround will be needed for current gens, lets update the comment
and remove the FIXME.

BSpec: 7723
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@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/20190406005112.27205-1-jose.souza@intel.com