drm/amd/display: Add null check before access structs

In enable_phantom_plane, we should better check null pointer before
accessing various structs.

Fixes: 09a4ec5da9 ("drm/amd/display: Refactor dc_state interface")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml2/dml2_mall_phantom.c

@@ -750,6 +750,8 @@ static void enable_phantom_plane(struct dml2_context *ctx,
 					ctx->config.svp_pstate.callbacks.dc,
 					state,
 					curr_pipe->plane_state);
+			if (!phantom_plane)
+				return;
 		}
 
 		memcpy(&phantom_plane->address, &curr_pipe->plane_state->address, sizeof(phantom_plane->address));