This reverts commit a7442b93.

With the patch applied SNB, IVB and ILK are experiencing hard machine
hangs. Original patch was to fix "just" kernel panics so it's not a good
trade-off.

Proper fix for the panic is on the way, lets revert until then.

Fixes: a7442b93 ("drm/i915: Fix races on fbdev")
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: stable@vger.kernel.org
Acked-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459510861-29035-1-git-send-email-joonas.lahtinen@linux.intel.com