drm/tegra: Use proper IOVA address for cursor image

The IOVA address for the cursor is the result of mapping the buffer
object for the given display controller. Make sure to use the proper
IOVA address as stored in the cursor's plane state.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>