drm/amdgpu/sdma5: init engine reset mutex

Missing the mutex init.

Fixes: e56d4bf5 ("drm/amdgpu/: drm/amdgpu: Register the new sdma function pointers for sdma_v5_0")
Reviewed-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>