drm/i2c: tda998x: power down pre-filter and color conversion

Disabling the pre-filter block of the TDA998x saves 40mW and the colour
conversion block saves 15mW.  As we always disable these two blocks, we
can power these sections of the chip down to save 55mW of unnecessary
power consumption.

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>