drm/vc4: hdmi: unlock mutex when device is unplugged

In vc4_hdmi_encoder_{pre,post}_crtc_enable() commit cd00ed51
("drm/vc4: hdmi: Protect device resources after removal") missed to
unlock the mutex before returning due to drm_dev_enter() indicating the
device being unplugged.

Fixes: cd00ed51 ("drm/vc4: hdmi: Protect device resources after removal")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220824161327.330627-2-dakr@redhat.com