drm/i915: Convert the register access tracepoint to be conditional

The TRACE_EVENT_CONDITION is supposed to generate more efficient code
than if (cond) trace(), which is what we are currently using inside the
register access functions.

v2: Rebase onto uncore

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