[SPARC]: Mark as emulating cmpxchg, add appropriate depends for DRM.

The DRM code depends on an atomic version of cmpxchg(), which is not
available on sparc32. Since other platforms besides sparc32 have this
issue a KCONFIG option is added for it.
Signed-off-by: Martin Habets <errandir_news@mph.eclipse.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/Kconfig

@@ -178,6 +178,13 @@ config ARCH_HAS_ILOG2_U64
 	bool
 	default n
 
+config EMULATED_CMPXCHG
+	bool
+	default y
+	help
+	  Sparc32 does not have a CAS instruction like sparc64. cmpxchg()
+	  is emulated, and therefore it is not completely atomic.
+
 config SUN_PM
 	bool
 	default y

drivers/char/drm/Kconfig

@@ -6,7 +6,7 @@
 #
 config DRM
 	tristate "Direct Rendering Manager (XFree86 4.1.0 and higher DRI support)"
-	depends on (AGP || AGP=n) && PCI
+	depends on (AGP || AGP=n) && PCI && !EMULATED_CMPXCHG
 	help
 	  Kernel-level support for the Direct Rendering Infrastructure (DRI)
 	  introduced in XFree86 4.0. If you say Y here, you need to select