The wptr needs to be incremented at at least 64 dword intervals,
use 256 to align with windows.  This should fix potential hangs
with unaligned updates.
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>