accel/ivpu: Do not access HW registers after unbind

We should not access hardware after we unbind from the bus.

Use drm_dev_enter() / drm_dev_exit() to mark code sections where
hardware is accessed (and not already protected by other locks)
and drm_dev_unplug() to mark device is gone.

Fixes: 35b13763 ("accel/ivpu: Introduce a new DRM driver for Intel VPU")
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323125504.2586442-2-stanislaw.gruszka@linux.intel.com