drm/i915: fix typo in display IRQ mask when disabling IRQs

Introduced in
commit e0e33f8ff6f0b6d286afc314802be4993341bd47
Author: Imre Deak <imre.deak@intel.com>
Date:   Tue Mar 4 19:23:07 2014 +0200

The impact was luckily minimal, due to the extra check we do against a
software pipestat IRQ mask.

Caught by Fengguang's 0-day tester.

Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>