drm/i915: add a tracepoint for gpu frequency changes

drm/i915: add a tracepoint for gpu frequency changes

We've had and still have too many issues where the gpu turbo doesn't
quite to what it's supposed to do (or what we want it to do).

Adding a tracepoint to track when the desired gpu frequency changes
should help a lot in characterizing and understanding problematic
workloads.

Also, this should be fairly interesting for power tuning (and
especially noticing when the gpu is stuck in high frequencies, as has
happened in the past) and hence for integration into powertop and
similar tools.

Cc: Arjan van de Ven <arjan@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>