Revert "drm/i915: Fix races on fbdev"

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