drm/core: Add drm_encoder_index.

This is useful for adding encoder_mask in crtc_state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452160762-30487-2-git-send-email-maarten.lankhorst@linux.intel.com