drm/i2c: tda998x: remove useless NULL checks

There is no way 'priv' can be NULL in tda998x_irq_thread() - this can
only happen if request_threaded_irq() was passed a NULL priv pointer,
and we would have crashed long before then if that was the case.

We also always ensure that priv->encoder is correctly setup, which
must have been initialised prior to the interrupt being claimed, so we
can remove this check as well.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>