drm/amdgpu: fix over allocating of IRQ sources

We need an array of pointers to IRQ sources, not an array of sources.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>