drm/vc4: Convert more lock requirement comments to lockdep assertions.

Since I do my development with lockdep on, this will help make sure I
don't introduce bugs here.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170725182718.31468-3-eric@anholt.net


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>