drm/amd/pm: fix variable dereferenced issue in amdgpu_device_attr_create()

- fix variable ('attr') dereferenced issue.
- using condition check instead of BUG_ON().

Fixes: 4e01847c ("drm/amdgpu: optimize amdgpu device attribute code")
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/amdgpu_pm.c

@@ -2171,15 +2171,19 @@ static int amdgpu_device_attr_create(struct amdgpu_device *adev,
 				     uint32_t mask, struct list_head *attr_list)
 {
 	int ret = 0;
-	struct device_attribute *dev_attr = &attr->dev_attr;
-	const char *name = dev_attr->attr.name;
 	enum amdgpu_device_attr_states attr_states = ATTR_STATE_SUPPORTED;
 	struct amdgpu_device_attr_entry *attr_entry;
+	struct device_attribute *dev_attr;
+	const char *name;
 
 	int (*attr_update)(struct amdgpu_device *adev, struct amdgpu_device_attr *attr,
 			   uint32_t mask, enum amdgpu_device_attr_states *states) = default_attr_update;
 
-	BUG_ON(!attr);
+	if (!attr)
+		return -EINVAL;
+
+	dev_attr = &attr->dev_attr;
+	name = dev_attr->attr.name;
 
 	attr_update = attr->attr_update ? attr->attr_update : default_attr_update;