drm/amdgpu/sdma4: set align mask to 255

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>