drm/i2c: tda998x: ensure VIP output mux is properly set

When switching between various drivers for this device, it's possible
that some critical registers are left containing values which affect
the device operation.  One such case encountered is the VIP output
mux register.  This defaults to 0x24 on powerup, but other drivers may
set this to 0x12.  This results in incorrect colours.

Fix this by ensuring that the register is always set to the power on
default setting.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Russell King <rmk_kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>