I want to also include kerneldoc from the header (for static inline
functions and structs), but fishing the right pieces out of a giant
header is a real pain. So split things out.

Note that it's not a really clean header with sane include orders, but
given's drm historical knack for giant headers detangling this is a
major task.

v2: Also extract struct drm_cmdline_mode.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>