drm/i2c: tda998x: allow interrupt to be shared

Some TDA998x contain several different I2C devices - there is the HDMI
encoder, and there is a TDA9950 CEC engine.  These two share the same
interrupt signal.

In order to allow a driver for the CEC engine to work, we need to be
able to share the interrupt with the CEC driver, so convert the handler
and registration to allow this to happen.

Tested-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>