drm: Document deprecated load/unload hook

v2:
 - Replace discouraged with deprecated
 - Link to new initialization/teardown functions

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170102142008.22174-3-krisman@collabora.co.uk