drm/amd/display: Remove underflow IRQ type

[WHY]
Feature using the underflow IRQ type reverted

[HOW]
Removed underflow IRQ type entry

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Angus Wang <Angus.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>