drm: Parse color format information in CEA blocks

The CEA extension block has a field which describes which YCbCr modes are
supported by the device, use it to fill the drm_display_info color_formats
fields. Also the existence of a CEA extension block is used as indication
that the device supports RGB.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>