amdgpu: Add explicit dependency test.

The test is as following:

1) Create context A & B
2) Send a command submission using context A which fires up a compute shader.
3) The shader wait a bit and then write a value to a memory location.
4) Send a command submission using context B which writes another value to the same memory location, but having an explicit dependency on the first command submission.
5) Wait with the CPU for both submissions to finish and inspect the written value.

Test passes if the value seen in the memory location after both submissions is from command B.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>