drm: Constify drm_encoder_slave_funcs

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-11-git-send-email-boris.brezillon@free-electrons.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>