drm/i915: Add debug info for failed MSI enabling

While not being able to enable MSI interrupts may be a normal
circumstance, for debugging it may still be a useful information, so
emit an info about this.
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NDavid Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454071949-24677-3-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/i915_dma.c

@@ -1109,8 +1109,10 @@ int i915_driver_load(struct drm_device *dev, unsigned long flags)
 	 * be lost or delayed, but we use them anyways to avoid
 	 * stuck interrupts on some machines.
 	 */
-	if (!IS_I945G(dev) && !IS_I945GM(dev))
-		pci_enable_msi(dev->pdev);
+	if (!IS_I945G(dev) && !IS_I945GM(dev)) {
+		if (pci_enable_msi(dev->pdev) < 0)
+			DRM_DEBUG_DRIVER("can't enable MSI");
+	}
 
 	intel_device_info_runtime_init(dev);