drm/i915: Re-enable self-refresh

A broken implementation of is_pot() prevented the detection of when a
singular pipe was enabled. Eric Anholt pointed out the existence of
is_power_of_2() so use that instead of our broken code!

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35402
Signed-off-by: Yuanhan Liu <[e-mail]>
Tested-by: [e-mail]
Reviewed-by: Keith Packard <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>

drivers/gpu/drm/i915/intel_display.c

···
 			      display, cursor);
 }
 
-static inline bool single_plane_enabled(unsigned int mask)
-{
-	return mask && (mask & -mask) == 0;
-}
+#define single_plane_enabled(mask) is_power_of_2(mask)
 
 static void g4x_update_wm(struct drm_device *dev)
 {