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

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>
(cherry picked from commit 4e6b1e91253ebf701b4ec3586cba5596527a4d0a)

[inki.dae: apply from rpi3-4.9.y]
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Change-Id: I51614f8783c7e022e873bc40bca31beabf4a9ad0