drm/plane: drop num_overlay_planes (v3)

In order to implement plane leasing we need to count things,
just make the code consistent with the counting code currently
used for counting crtcs/encoders/connectors and drop the need
for num_overlay_planes.

v2: don't forget to assign plane_ptr. (keithp)
v3: use correct bounds check, found by igt.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>