drm/vc4: plane: Create more planes

Let's now create more planes that can be affected to all the CRTCs.

vc4 has 3 CRTCs, 1 primary and 1 cursor each, and was having 24 (8
planes per CRTC) overlays.

However, vc5 has 5 CRTCs, so keeping the same logic would put us at 50
planes which is well above the 32 planes limit imposed by DRM.

Using 16 seems like a good tradeoff between staying under 32 and yet
providing enough planes.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/b41003001541fc2bb23668c699c0369ff7983be8.1599120059.git-series.maxime@cerno.tech