drm/i915: Add locking to psr code

drm/i915: Add locking to psr code

It's not really optional to have locking ...

The ugly part is how much locking the psr work needs since it has to
recheck everything. Which is way too much. But we need to ditch the
psr work in it's current form anyway and implement proper frontbuffer
tracking.

The other nasty bit that had to go was the delayed work cancle in
psr_exit. Which means a bunch of races just became a bit more likely,
but mea culpa.

v2: Fixup HAS_PSR checks, resulting in uninitialized mutex issues.

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