drm/i2c: tda998x: handle all outstanding interrupts

As reading the interrupt registers clears the outstanding interrupts, we
must process all received interrupts to avoid dropping any.  Rearrange
the code to achieve this, and properly check for a HPD interrupt from
the CEC_RXSHPDINT register.

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