drm/i915: properly clear IIR at irq_uninstall on Gen5+

The IRQ_INIT and IRQ_FINI macros are basically the same thing, with
the exception that IRQ_FINI doesn't properly clear IIR twice and
doesn't have as many POSTING_READs as IRQ_INIT. So rename the INIT
macro to IRQ_RESET and use it everywhere.

v2: - Fix error in the commit message (Chris).
    - Adjust to the new POSTING_READ scheme (Ben).

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