drm/i915: Drop schedule_back from psr_exit

It doesn't make sense to never again schedule the work, since by the
time we might want to re-enable psr the world might have changed and
we can do it again.

The only exception is when we shut down the pipe, but that's an
entirely different thing and needs to be handled in psr_disable.

Note that later patch will again split psr_exit into psr_invalidate
and psr_flush. But the split is different and this simplification
helps with the transition.

v2: Improve the commit message a bit.

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