drm/i915/display: Do not write in removed FBC fence registers

Platforms without fences don't have FBC host tracking and those
registers are marked as reserved in those platforms.

v2: checking num_fences to write to FBC fence registers (Ville)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306185833.53984-2-jose.souza@intel.com

drivers/gpu/drm/i915/display/intel_fbc.c

@@ -321,7 +321,7 @@ static void gen7_fbc_activate(struct drm_i915_private *dev_priv)
 			       SNB_CPU_FENCE_ENABLE | params->fence_id);
 		intel_de_write(dev_priv, DPFC_CPU_FENCE_OFFSET,
 			       params->crtc.fence_y_offset);
-	} else {
+	} else if (dev_priv->ggtt.num_fences) {
 		intel_de_write(dev_priv, SNB_DPFC_CTL_SA, 0);
 		intel_de_write(dev_priv, DPFC_CPU_FENCE_OFFSET, 0);
 	}