drm/display: use ERR_PTR on DP tunnel manager creation fail

Instead of returning a generic NULL on error from
drm_dp_tunnel_mgr_create(), use error pointers with informative codes
to align the function with stub that is executed when
CONFIG_DRM_DISPLAY_DP_TUNNEL is unset. This will also trigger IS_ERR()
in current caller (intel_dp_tunnerl_mgr_init()) instead of bypassing it
via NULL pointer.

v2: use error codes inside drm_dp_tunnel_mgr_create() instead of handling
 on caller's side (Michal, Imre)

v3: fixup commit message and add "CC"/"Fixes" lines (Andi),
 mention aligning function code with stub

Fixes: 91888b5 ("drm/i915/dp: Add support for DP tunnel BW allocation")
Cc: Imre Deak <[email protected]>
Cc: <[email protected]> # v6.9+
Signed-off-by: Krzysztof Karas <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/7q4fpnmmztmchczjewgm6igy55qt6jsm7tfd4fl4ucfq6yg2oy@q4lxtsu6445c

Author: KKaras169169

drivers/gpu/drm/display/drm_dp_tunnel.c

@@ -1896,8 +1896,8 @@ static void destroy_mgr(struct drm_dp_tunnel_mgr *mgr)
  *
  * Creates a DP tunnel manager for @dev.
  *
- * Returns a pointer to the tunnel manager if created successfully or NULL in
- * case of an error.
+ * Returns a pointer to the tunnel manager if created successfully or error
+ * pointer in case of failure.
  */
 struct drm_dp_tunnel_mgr *
 drm_dp_tunnel_mgr_create(struct drm_device *dev, int max_group_count)
@@ -1907,7 +1907,7 @@ drm_dp_tunnel_mgr_create(struct drm_device *dev, int max_group_count)
 
 	mgr = kzalloc(sizeof(*mgr), GFP_KERNEL);
 	if (!mgr)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	mgr->dev = dev;
 	init_waitqueue_head(&mgr->bw_req_queue);
@@ -1916,7 +1916,7 @@ drm_dp_tunnel_mgr_create(struct drm_device *dev, int max_group_count)
 	if (!mgr->groups) {
 		kfree(mgr);
 
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 	}
 
 #ifdef CONFIG_DRM_DISPLAY_DP_TUNNEL_STATE_DEBUG
@@ -1927,7 +1927,7 @@ drm_dp_tunnel_mgr_create(struct drm_device *dev, int max_group_count)
 		if (!init_group(mgr, &mgr->groups[i])) {
 			destroy_mgr(mgr);
 
-			return NULL;
+			return ERR_PTR(-ENOMEM);
 		}
 
 		mgr->group_count++;