drm/i915: Limit calling mark-busy only for potential scanouts

The principle of intel_mark_busy() is that we want to spot the
transition of when the display engine is being used in order to bump
powersaving modes and increase display clocks. As such it is only
important when the display is changing, i.e. when rendering to the
scanout or other sprite/plane, and these are characterised by being
pinned.

v2: Mark the whole device as busy on execbuffer and pageflips as well
and rebase against dinq for the minor bug fix to be immediately
applicable.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: fix compile fail.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>