drm/i915: only enable sdvo hotplug irq if needed

commit fcbc50da7753b210b4442ca9abc4efbd4e481f6e upstream.

Avoid constant wakeups caused by noisy irq lines when we don't even care
about the irq. This should be particularly useful for i945g/gm where the
hotplug has been disabled:

commit 768b107e4b3be0acf6f58e914afe4f337c00932b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri May 4 11:29:56 2012 +0200

    drm/i915: disable sdvo hotplug on i945g/gm

v2: While at it, remove the bogus hotplug_active read, and do not mask
hotplug_active[0] before checking whether the irq is needed, per discussion
with Daniel on IRC.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=38442
Tested-by: Dominik Köppl <dominik@devwork.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>