drm/amd/display: Fix pointer dereferenced before checking

The pointer edid_buf is dereferencing pointer edid before edid is being
 null checked. Fix this by assigning edid->raw_edid to edid_buf only if
edid is not NULL, otherwise just NULL.
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -90,7 +90,7 @@ enum dc_edid_status dm_helpers_parse_edid_caps(
 {
 	struct amdgpu_dm_connector *aconnector = link->priv;
 	struct drm_connector *connector = &aconnector->base;
-	struct edid *edid_buf = (struct edid *) edid->raw_edid;
+	struct edid *edid_buf = edid ? (struct edid *) edid->raw_edid : NULL;
 	struct cea_sad *sads;
 	int sad_count = -1;
 	int sadb_count = -1;