We should only enable interrupts at postinstall.

And now on ILK/SNB/IVB/HSW the irq_preinstall and irq_postinstall
functions leave the hardware in the same state.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix compile fail due to drm_i915_private_t typedef removal.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>