Partially revert "drm/i915: unconditionally use mt forcewake on hsw/ivb"

This patch partially reverts commit 36ec8f87 for
IvyBridge CPUs.

The original commit results in repeated 'Timed out waiting for forcewake old
ack to clear' messages on a Supermicro C7H61 board (BIOS version 2.00 and 2.00b)
with i7-3770K CPU. It ultimately results in a hangup if the system is highly
loaded. Reverting the commit for IvyBridge CPUs fixes the issue.

Issue a warning if the CPU is IvyBridge and mt forcewake is disabled, since
this condition can result in secondary issues.

v2: Only revert patch for Ivybridge CPUs
    Issue info message if mt forcewake is disabled on Ivybridge

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=60541


Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66139


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>