drm: add helper to check for wc memory support

BugLink: http://bugs.launchpad.net/bugs/1553179

commit 4b0e4e4a upstream.
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

include/drm/drm_cache.h

@@ -35,4 +35,13 @@
 
 void drm_clflush_pages(struct page *pages[], unsigned long num_pages);
 
+static inline bool drm_arch_can_wc_memory(void)
+{
+#if defined(CONFIG_PPC) && !defined(CONFIG_NOT_COHERENT_CACHE)
+	return false;
+#else
+	return true;
+#endif
+}
+
 #endif