drm/amdgpu: Call pageflip irq funtion when receiced vsync interrupt.

For virtual display feature, as there is no dce engine, so no pageflip irq
generated. So directly call pageflip irq funtion when received vysn 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>