drm: drm_connector->s/connector_id/index/ for consistency

connector_id in the uapi actually means drm_connector->base.id, which
is something entirely different. And ->index is also consistent with
plane/encoder/CRTCS and the various drm_*_index() functions.

While at it also improve/align the kerneldoc comment.

v2: Mention where those ids are from ...

v3: Add -ing to supporting and try to not break the world.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468945501-23166-1-git-send-email-daniel.vetter@ffwll.ch