drm/i915: fix the SDE irq dmesg warnings properly

drm/i915: fix the SDE irq dmesg warnings properly

We had the "The master control interrupt lied (SDE)!" check and error
message in place for a long time without any problems, until

commit 42fc36df527c600c55e83f4117f93b29df9e81cf
Author: Sonika Jindal <sonika.jindal@intel.com>
Date:   Wed Jul 8 17:07:47 2015 +0530

    drm/i915: Handle HPD when it has actually occurred

caused the errors to start happening. This was bisected and reported,
but the error message was silenced in

commit c90fd5fc0fe377cedd6caa936067f794d46042d3
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Oct 23 10:56:12 2015 +0200

    drm/i915: shut up gen8+ SDE irq dmesg noise

shooting the messenger while the debugging for why Sonika's commit
triggered the errors was still in progress.

It looks like we need to read and acknowledge the PCH_PORT_HOTPLUG
register even though the hotplug trigger indicates there isn't a hotplug
irq to handle. The PCH doesn't seem to really ack the the interrupt to
the CPU unless we touch the hotplug register.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92084
Fixes: 42fc36df527c ("drm/i915: Handle HPD when it has actually occurred")
[Jani: added a comment and amended the commit message while applying]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448462843-32739-1-git-send-email-jani.nikula@intel.com