drm/tegra: Track active planes in CRTC state

Wrap struct drm_crtc_state in a driver-specific structure and add the
planes field which keeps track of which planes are updated or disabled
during a modeset. This allows atomic updates of the the display engine
at ->atomic_flush() time.

v2: open-code getting the state of the CRTC that the plane is being
    attached to (Daniel Vetter)

Signed-off-by: Thierry Reding <treding@nvidia.com>