drm/i915: Use new frontbuffer bits to increase pll clock

The downclocking checks a few more things, so not that simple to
convert. Also, this should get unified with the drrs handling and also
use the locking of that. Otoh the drrs locking is about as hapzardous
as no locking, at least on first sight.

For easier conversion ditch the upclocking on unload - we'll turn off
everything anyway.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>