This fixes a regression introduced in

commit e5868a31
Author: Egbert Eich <eich@suse.de>
Date:   Thu Feb 28 04:17:12 2013 -0500

    DRM/i915: Convert HPD interrupts to make use of HPD pin assignment in encode

Due to the irq setup rework in 3.9, see

commit 20afbda2
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Dec 11 14:05:07 2012 +0100

    drm/i915: Fixup hpd irq register setup ordering

Egbert Eich's hpd rework blows up on pch-split platforms - it walks
the encoder list before that has been set up completely. The new init
sequence is:

1. irq enabling
2. modeset init
3. hpd setup

We need to move around the ibx setup a bit to fix this.

Ville Syrjälä pointed out in his review that we can't touch SDEIER
after the interrupt handler is set up, since that'll race with Paulo
Zanoni's PCH interrupt race fix:

commit 44498aea
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Feb 22 17:05:28 2013 -0300

    drm/i915: also disable south interrupts when handling them

We fix that by unconditionally enabling all interrupts in SDEIER, but
masking them as-needed in SDEIMR. Since only the single-threaded
setup/teardown (or suspend/resume) code touches that, no further
locking is required.

While at it also simplify the mask handling - we start out with all
interrupts cleared in the postinstall hook, and never enable a hpd
interrupt before hpd_irq_setup is called.

And finally, for consistency rename the ibx hpd setup function to
ibx_hpd_irq_setup.

v2: Fix race around SDEIER writes (Ville).

v3: Remove the superflous posting read for SDEIER, spotted by Ville.

Ville also wondered whether we shouldn't clear SDEIIR, since now
SDE interrupts are enabled before we have an irq handler installed.
But the master interrupt control bit in DEIER is still cleared, so we
should be fine.

Cc: Egbert Eich <eich@suse.de>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62798Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>