ARC PGU doesn't support RGB888 (24 bit) format but supports
XRGB888 (32 bit) format. Fix incorrect format list in a driver.
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>