drm/amd/display: invalid parameter check in dmub_hpd_callback

commit 978ffac878 upstream.

The function performs a check on the "adev" input parameter, however, it
is used before the check.

Initialize the "dev" variable after the sanity check to avoid a possible
NULL pointer dereference.

Fixes: e27c41d5b0 ("drm/amd/display: Support for DMUB HPD interrupt handling")
Addresses-Coverity-ID: 1493909 ("Null pointer dereference")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -653,7 +653,7 @@ void dmub_hpd_callback(struct amdgpu_device *adev, struct dmub_notification *not
 	struct drm_connector_list_iter iter;
 	struct dc_link *link;
 	uint8_t link_index = 0;
-	struct drm_device *dev = adev->dm.ddev;
+	struct drm_device *dev;
 
 	if (adev == NULL)
 		return;
@@ -670,6 +670,7 @@ void dmub_hpd_callback(struct amdgpu_device *adev, struct dmub_notification *not
 
 	link_index = notify->link_index;
 	link = adev->dm.dc->links[link_index];
+	dev = adev->dm.ddev;
 
 	drm_connector_list_iter_begin(dev, &iter);
 	drm_for_each_connector_iter(connector, &iter) {