drm/i2c: tda998x: free the CEC device on encoder_destroy

The cec i2c device is created in tda998x_encoder_init() when the DRM
driver starts.
This patch frees it when the DRM driver is unloaded.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>