drm/amd: use kmalloc_array over kmalloc with multiply

Fix patch check warning:
WARNING: Prefer kmalloc_array over kmalloc with multiply
+	buf = kmalloc(MAX_KFIFO_SIZE * sizeof(*buf), GFP_KERNEL);

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>