drm/crtc: constify drm_crtc_index parameter

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476113170-13816-1-git-send-email-jani.nikula@intel.com