Atm, it's possible that the interrupt handler is called when the device
is in D3 or some other low-power state. It can be due to another device
that is still in D0 state and shares the interrupt line with i915, or on
some platforms there could be spurious interrupts even without sharing
the interrupt line. The latter case was reported by Klaus Ethgen using a
Lenovo x61p machine (gen 4). He noticed this issue via a system
suspend/resume hang and bisected it to the following commit:

commit e11aa362
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Wed Jun 18 09:52:55 2014 -0700

    drm/i915: use runtime irq suspend/resume in freeze/thaw

This is a problem, since in low-power states IIR will always read
0xffffffff resulting in an endless IRQ servicing loop.

Fix this by handling interrupts only when the driver explicitly enables
them and so it's guaranteed that the interrupt registers return a valid
value.

Note that this issue existed even before the above commit, since during
runtime suspend/resume we never unregistered the handler.

v2:
- clarify the purpose of smp_mb() vs. synchronize_irq() in the
  code comment (Chris)

v3:
- no need for an explicit smp_mb(), we can assume that synchronize_irq()
  and the mmio read/writes in the install hooks provide for this (Daniel)
- remove code comment as the remaining synchronize_irq() is self
  explanatory (Daniel)

v4:
- drm_irq_uninstall() implies synchronize_irq(), so no need to call it
  explicitly (Daniel)

Reference: https://lkml.org/lkml/2015/2/11/205Reported-and-bisected-by: Klaus Ethgen <Klaus@Ethgen.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>