drm/i915: Fix irq enable tracking in driver load

A bunch of warnings fire on some ->irq_postinstall hooks since those
can enable interrupts (e.g. rps interrupts). And then our ordering
self-checks fire and complain.

To fix that set the tracking boolen before enabling the irqs with
drm_irq_install. Quoting the discussion with Jesse why that's safe:

On Tue, Aug 26, 2014 at 11:18 PM, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> Yes, it might work, but if you look through the history, we set this
> field carefully; first to true in the irq_init code, then to false only
> after the irq_install completes.  So I think your fragility arguments
> apply to this change too.

Well we've done it in 4 commits or so, but currently we have:

- Set irqs_disabled to true early in driver load to make sure checks
that. That's done in irq_init, which is totally not the function that
enables interrupts, only the function that initializes all the vtables
and similar things. We actually have a fairly sane naming scheme
nowadays (not fully consistent ofc): _init is sw setup,
_enable/_hw_init is the actual hw setup. That is done in
95f25bed

- Set irqs_disabled to false right after the irqs are actually
enabled. This is done in ed2e6df1

So my change should only move the flag change over the ->preinstall
and ->postinstall hooks. I've done a little audit and didn't spot
anything amiss. Furthermore the runtime pm setup already clears
irqs_disabled _before_ calling these two hooks.

This regression has been introduced in

commit ed2e6df1
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Jun 20 09:39:36 2014 -0700

    drm/i915: clear pm._irqs_disabled field after installing IRQs

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Oliver Hartkopp <socketcan@hartkopp.net>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk> # gm45, ilk
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>