drm/omapdrm: Fix omap_atomic_wait_for_completion

Use the new iterator macro and look for crtc_state->active instead of
enable, only crtc_state->active implies that vblanks will happen.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719143920.25685-5-maarten.lankhorst@linux.intel.com