drm/i915: do not wait for vblank when crtc is off

This can happen when turning off a sprite plane. Because the crtc state
is not yet always swapped correctly and transitional helpers are used
the crtc state cannot be relied on.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>