drm/amdgpu: avoid dereferencing a NULL pointer

[ Upstream commit 55611b50 ]

Check if irq_src is NULL to avoid dereferencing a NULL pointer,
for MES ring is uneccessary to recieve an interrupt notification.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>