drm/i915: Lock down psr sw/hw state tracking

Make sure we track the sw side (psr.active) correctly and WARN
everywhere it might get out of sync with the hw.

v2: Fixup WARN_ON logic inversion, reported by Rodrigo.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>