DRM: ARC: PGU: cleanup supported format list code

Get rid of 'simplefb_format' structure usage as we only use its
'fourcc' field.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>