drm/i915: Treat crtc->mode.clock == 0 as disabled

drm/i915: Treat crtc->mode.clock == 0 as disabled

Prevent a divide-by-zero by consistently treating an 'active' CRTC
without a mode set as actually disabled.

This looks to have been first introduced with

commit 464edc281c3319d06c635f5c1979807393ec7fc3
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Jul 2 20:28:59 2012 +0200

    drm/i915: read out the modeset hw state at load and resume time

but then combined with

commit 69a44eaa480894b98e6e720dffd91945afe631dc
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Dec 18 09:37:54 2012 +0100

    drm/i915: don't disable disconnected outputs

it finally started oopsing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>