drm/i915: add gen8_irq_reset

So we can merge all the common code from postinstall and uninstall.

v2: - Rebase.
    - While at it, remove useless { and }.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>