drm/vc4: crtc: Move the cob allocation outside of bind

The COB allocation depends on the HVS channel used for a given
pixelvalve.

While the channel allocation was entirely static in vc4, vc5 changes
that and at bind time, a pixelvalve can be assigned to multiple
HVS channels.

Let's prepare that rework by allocating the COB when it's actually
needed.

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/484cbd4b00cfeee425295df438222258cc39a3dd.1599120059.git-series.maxime@cerno.tech