drm/i2c: tda998x: improve correctness of quantisation range

CEA-861 says: "A Source shall not send a non-zero Q value that does
not correspond to the default RGB Quantization Range for the
transmitted Picture unless the Sink indicates support for the Q bit
in a Video Capabilities Data Block."

Make TDA998x compliant by using the helper to set the quantisation
range in the infoframe, and using the TDA998x's colour scaling to
appropriately adjust the RGB values sent to the monitor.

This ensures that monitors that do not support the Q bit are sent
RGB values that are within the expected range.  Monitors with
support for the Q bit will be sent full-range RGB.

Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Tested-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>