drm/i915: disable sdvo hotplug on i945g/gm

Chris Wilson dug out a hw erratum saying that there's noise on the
interrupt line on i945G chips. We also have a bug report from a i945GM
chip with an sdvo hotplug interrupt storm (and no apparent cause).

Play it safe and disable sdvo hotplug on all i945 variants.

Note that this is a regression that has been introduced in 3.1,
when we've enabled sdvo hotplug support with

commit cc68c81
Author: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Date:   Wed Sep 21 17:13:30 2011 +0100

    drm/i915: Enable SDVO hotplug interrupts for HDMI and DVI

Cc: stable@kernel.org
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=38442
Reported-and-tested-by: Dominik Köppl <dominik@devwork.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_sdvo.c

@@ -1220,8 +1220,14 @@ static bool intel_sdvo_get_capabilities(struct intel_sdvo *intel_sdvo, struct in
 
 static int intel_sdvo_supports_hotplug(struct intel_sdvo *intel_sdvo)
 {
+	struct drm_device *dev = intel_sdvo->base.base.dev;
 	u8 response[2];
 
+	/* HW Erratum: SDVO Hotplug is broken on all i945G chips, there's noise
+	 * on the line. */
+	if (IS_I945G(dev) || IS_I945GM(dev))
+		return false;
+
 	return intel_sdvo_get_value(intel_sdvo, SDVO_CMD_GET_HOT_PLUG_SUPPORT,
 				    &response, 2) && response[0];
 }