drm/mediatek: Fix mtk_atomic_complete for runtime_pm

To properly implement atomic w/ runtime pm, we move
drm_atomic_helper_commit_modeset_enables() above
drm_atomic_helper_commit_planes() to ensure CRTCs are enabled before
modifying plane registers, and set active_only to true to filter out
plane update notifications when the CRTC is disabled.

According to the document from linux kernel:
Set the active_only parameters to true in order not to receive plane
update notifications related to a disabled CRTC. This avoids the need
to manually ignore plane updates in driver code when the driver and/or
hardware can't or just don't need to deal with updates on disabled
CRTCs, for example when supporting runtime PM.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470279597-60453-8-git-send-email-bibby.hsieh@mediatek.com