drm/i915/psr: Restrict single-shot updates to the PSR pipe

The frontbuffer code gives us accurate information about activity,
let's use it. Again this should avoid unecessary updates when multiple
screens are on.

Also realign function paramaters, I couldn't resist that bit of OCD.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>