drm: Add retries for lspcon mode detection

From the CI builds, its been observed that during a driver
reload/insert, dp dual mode read function sometimes fails to
read from LSPCON device over i2c-over-aux channel.

This patch:
- adds some delay and few retries, allowing a scope for these
  devices to settle down and respond.
- changes one error log's level from ERROR->DEBUG as we want
  to call it an error only after all the retries are exhausted.

V2: Addressed review comments from Jani (for loop for retry)
V3: Addressed review comments from Imre (break on partial read too)
V3: Addressed review comments from Ville/Imre (Add the retries
    exclusively for LSPCON, not for all dp_dual_mode devices)
V4: Added r-b from Imre, sending it to dri-devel (Jani)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102294
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102295
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102359
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103186
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507826408-19322-1-git-send-email-shashank.sharma@intel.com

drivers/gpu/drm/drm_dp_dual_mode_helper.c

@@ -410,6 +410,7 @@ int drm_lspcon_get_mode(struct i2c_adapter *adapter,
 {
 	u8 data;
 	int ret = 0;
+	int retry;
 
 	if (!mode) {
 		DRM_ERROR("NULL input\n");
@@ -417,10 +418,19 @@ int drm_lspcon_get_mode(struct i2c_adapter *adapter,
 	}
 
 	/* Read Status: i2c over aux */
-	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_LSPCON_CURRENT_MODE,
-				    &data, sizeof(data));
+	for (retry = 0; retry < 6; retry++) {
+		if (retry)
+			usleep_range(500, 1000);
+
+		ret = drm_dp_dual_mode_read(adapter,
+					    DP_DUAL_MODE_LSPCON_CURRENT_MODE,
+					    &data, sizeof(data));
+		if (!ret)
+			break;
+	}
+
 	if (ret < 0) {
-		DRM_ERROR("LSPCON read(0x80, 0x41) failed\n");
+		DRM_DEBUG_KMS("LSPCON read(0x80, 0x41) failed\n");
 		return -EFAULT;
 	}