drm/amdgpu: Add otg vertical IRQ Source

[Why & How]
In order to get appropriate timing for registers which
read/write is vertical line sensitive, add new IRQ source variable.
This interrupt is triggered by specific vertical line,

Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>