drm/i2c: tda998x: allocate tda998x_priv inside tda998x_create()

Move the tda998x_priv allocation inside tda998x_create() and simplify
the tda998x_create()'s arguments.  Pass the same to tda998x_destroy().

Tested-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>