drm/i915: Share crtc setup and teardown between dpms and disable/enable

This closes a couple of corner cases where we introduced and forgot
about a couple of routines that need to be called when disabling the
crtc and then re-enabling it. The code needs to be moved again so that
the common bits are shared across generations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>