drm/amd/display: add new IRQ enum for underflows

drm/amd/display: add new IRQ enum for underflows

[WHY & HOW]
needed in certain scenarios for debugging and logging

Reviewed-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Martin Leung <martin.leung@amd.com>
Signed-off-by: Leo Zeng <Leo.Zeng@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>