drm/i915: add fbc enable flag, but disable by default

FBC has too many corner cases that we don't currently deal with, so
disable it by default so we can enable more important features like RC6,
which conflicts in some configurations.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31742
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1065,6 +1065,9 @@ static int i915_fbc_status(struct seq_file *m, void *unused)
 		case FBC_MULTIPLE_PIPES:
 			seq_printf(m, "multiple pipes are enabled");
 			break;
+		case FBC_MODULE_PARAM:
+			seq_printf(m, "disabled per module param (default off)");
+			break;
 		default:
 			seq_printf(m, "unknown reason");
 		}