drm: fix drm_modeset_lock.h kernel-doc notation

Fix drm kernel-doc notation to squelch these warnings:

Warning(..//include/drm/drm_modeset_lock.h:41): cannot understand function prototype: 'struct drm_modeset_acquire_ctx '
Warning(..//include/drm/drm_modeset_lock.h:66): cannot understand function prototype: 'struct drm_modeset_lock '

Need to include the keyword 'struct' for structure descriptions.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>