drm: fourcc: Convert drm_format_info kerneldoc to in-line member documentation

drm: fourcc: Convert drm_format_info kerneldoc to in-line member documentation

In-line member documentation seems to be desired way of documenting
structure members.

This change had been suggested by Daniel Vetter here:
https://lists.freedesktop.org/archives/dri-devel/2018-October/192176.html

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181019105752.17741-2-alexandru-cosmin.gheorghe@arm.com