drm/amdgpu: Fix SDMA engine reset logic

The scheduler should restart only if the reset operation
succeeds  This ensures that new tasks are only submitted
to the queues after a successful reset.

Fixes: 9b5d6672 ("drm/amdgpu: Introduce conditional user queue suspension for SDMA resets")

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jesse.Zhang <Jesse.zhang@amd.com>
Reviewed-by: Tim Huang <tim.huang@amd.com>