drm/i915: Pass plane to vlv_compute_drain_latency()

Now that we have drm_planes for the cursor and primary we can move the
pixel_size handling into vlv_compute_drain_latency() and just pass the
appropriate plane to it.

v2: Check plane->state->fb instead of plane->fb

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v1)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Resolve conflict with Matt's s/plane->fb/plane->state->fb/
patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>