drm/amd/display: Create underflow interrupt IRQ type

[WHY]
We want another entry in IRQ type that can be used to
help find the underflow interrupt source.

[HOW]
Added another mapping in IRQ type enum.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Angus Wang <Angus.Wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>