drm/i915/dp: Helper for checking DDI_BUF_CTL Idle status

Modify the helper to add a fixed delay or poll with timeout
based on platform specification to check for either Idle bit
set (DDI_BUF_CTL is idle for disable case)

v2:
* Use 2 separate functions or idle and active (Ville)
v3:
* Change the timeout to 16usecs (Ville)
v4:
* Change the timeout 8, follow spec (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701221052.8946-1-manasi.d.navare@intel.com

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

@@ -1184,15 +1184,14 @@ static void intel_prepare_hdmi_ddi_buffers(struct intel_encoder *encoder,
 static void intel_wait_ddi_buf_idle(struct drm_i915_private *dev_priv,
 				    enum port port)
 {
-	i915_reg_t reg = DDI_BUF_CTL(port);
-	int i;
-
-	for (i = 0; i < 16; i++) {
-		udelay(1);
-		if (intel_de_read(dev_priv, reg) & DDI_BUF_IS_IDLE)
+	if (IS_BROXTON(dev_priv)) {
+		udelay(16);
 		return;
 	}
-	drm_err(&dev_priv->drm, "Timeout waiting for DDI BUF %c idle bit\n",
+
+	if (wait_for_us((intel_de_read(dev_priv, DDI_BUF_CTL(port)) &
+			 DDI_BUF_IS_IDLE), 8))
+		drm_err(&dev_priv->drm, "Timeout waiting for DDI BUF %c to get idle\n",
 			port_name(port));
 }