drm/vc4: drv: Use drm_dev_unplug

When our KMS driver is unbound, the device is no longer there but we might
still have users with an opened fd to the KMS device.

To avoid any issue in such a situation, every device access needs to be
protected by calls to drm_dev_enter() and drm_dev_exit(), and the driver
needs to call drm_dev_unplug().

We'll add calls to drm_dev_enter()/drm_dev_exit() in subsequent patches
changing the relevant drivers, but let's start by calling drm_dev_unplug().

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-14-maxime@cerno.tech