drm/i915/crt: convert to encoder disable/enable

CRT is the first output which can be cloned, hence we cannot (yet)
move the dpms handling over to disable/enable. This requires some more
smarts in intel_crtc_dpms first to set the display pipe status
depening upon encoder->connectors_active of all connected encoders.

Because that will happen in a separate step, don't touch the dpms
functions, yet.

v2: Be careful about clearing the _DISABLE flags for intermediate dpms
modes - otherwise we might clobber the crt state when another (cloned)
connector gets enabled.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>