drm/vc4: crtc: protect device resources after removal

(Hardware) resources which are bound to the driver and device lifecycle
must not be accessed after the device and driver are unbound.

However, the DRM device isn't freed as long as the last user closed it,
hence userspace can still call into the driver.

Therefore protect the critical sections which are accessing those
resources with drm_dev_enter() and drm_dev_exit().

Fixes: 7cc4214c ("drm/vc4: crtc: Switch to drmm_kzalloc")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220824161327.330627-4-dakr@redhat.com