x86: enable DMAR by default

Now that the obvious bugs have been worked out, specifically
the iwlagn issue, and the write buffer errata, DMAR should be safe
to turn back on by default. (We've had it on since those patches were
first written a few weeks ago, without any noticeable bug reports
(most have been due to the dma-api debug patchset.))

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/Kconfig

@@ -1803,7 +1803,7 @@ config DMAR
 	  remapping devices.
 
 config DMAR_DEFAULT_ON
-	def_bool n
+	def_bool y
 	prompt "Enable DMA Remapping Devices by default"
 	depends on DMAR
 	help