drm/amdgpu/sdma5.2: init engine reset mutex

Missing the mutex init.

Fixes: 47454f2d ("drm/amdgpu: Register the new sdma function pointers for sdma_v5_2")
Reviewed-by: Jesse Zhang <Jesse.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ea685ff3)