drm/atomic: Move the drm_atomic_state field doc inline

Some fields of drm_atomic_state have been documented in-line, but some
were documented in the main kerneldoc block before the structure.

Since the former is the preferred option in DRM, let's move all the
fields to an inline documentation.

Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://lore.kernel.org/r/20231214100917.277842-1-mripard@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>