drm/i915: Exit early from psr_status if PSR is not supported by the device

Static analysis was complaining that a path existed where we could use
stat[] uninitialized. Fix this by simplifying the logic to exit early if
PSR isn't supported.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>