drm/i2c: tda998x: correct PLL divider calculation

The serializer PLL divider is a power-of-two divider, so our calculation
which assumes that it's a numerical divider is incorrect.  Replace it
with one that results in a power-of-two divider value instead.

Tested with all supported modes with a Samsung S24C750.

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