drm/amdgpu: fix return random value when multiple threads read registers via mes.

The currect code use the address "adev->mes.read_val_ptr" to
store the value read from register via mes.
So when multiple threads read register,
multiple threads have to share the one address,
and overwrite the value each other.

Assign an address by "amdgpu_device_wb_get" to store register value.
each thread will has an address to store register value.

Signed-off-by: chongli2 <chongli2@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>