drm/i915: Require fence only for FBC capable planes

As only a subset of primary planes are FBC capable there's no need
to waste fences on all of them. So let's skip the fence if the plane
isn't even fbc capable.

In the future we might extend this to skip the fence even for FBC
capable planes if the crtc and/or plane state isn't suitable
for FBC.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-5-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -2072,7 +2072,7 @@ static bool intel_plane_uses_fence(const struct intel_plane_state *plane_state)
 	struct intel_plane *plane = to_intel_plane(plane_state->base.plane);
 	struct drm_i915_private *dev_priv = to_i915(plane->base.dev);
 
-	return INTEL_GEN(dev_priv) < 4 || plane->id == PLANE_PRIMARY;
+	return INTEL_GEN(dev_priv) < 4 || plane->has_fbc;
 }
 
 struct i915_vma *