drm/i915: Restrict GPU boost to the RCS engine

Make the assumption that media workloads are not as latency sensitive
for __wait_seqno, and that upclocking the GPU does not affect the BLT
engine. Under that assumption, we only wait to forcibly upclock the GPU
when we are stalling for results from the render pipeline.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Deepak <S&lt;deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>