drm/amdgpu: Use software timer to generate vsync interrupt.

For virtual display feature, use the software timer to
simulate the vsync interrupt.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>