drm/i915: Wait for pipe to start on i830 as well

We should make sure the pipe has fully started when we enable it from
the i830 "power well". Otherwise theoretically i830 could also hit
problems with vblank timestamps jumping around (since we skip the
wait during modeset on i830). Additionally moving planes between the
pipes etc. might not work correctly until both pipes are actually up and
running.

v2: Less pointless duplication in the code (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129153732.3612-2-ville.syrjala@linux.intel.com