drm/amdkfd: Remove vla

There's an ongoing effort to remove VLAs[1] from the kernel to eventually
turn on -Wvla. Switch to a constant value that covers all hardware.

[1] https://lkml.org/lkml/2018/3/7/621



Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>