drm: Constify drm_driver in drivers that don't modify it

A non-const structure containing function pointers is a possible attack
vector. The drm_driver structure is already const in most drivers, but
there are a few exceptions. Constify the structure in the drivers that
don't need to modify at, as a low-hanging fruit. The rest of the drivers
will need a more complex fix.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>