drm/atomic: Use active instead of enable in wait_for_vblanks.

When DPMS was introduced to atomic, vblanks only worked when the crtc
was enabled and active. wait_for_vblanks were not converted to check for
crtc_state->active, which may cause an attempt for vblank_get to fail.

This is probably harmless, but convert from enable to active anyway.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481204729-9058-2-git-send-email-maarten.lankhorst@linux.intel.com