drm/amd/display: Add Underflow Asserts to dc

[Why]
For debugging underflow issues it can be useful to have asserts when the
underflow initially occurs.

[How]
Read the underflow status registers after actions that have a high risk
of causing underflow and assert that no underflow occurred. If underflow
occurred, clear the bit.

Signed-off-by: Thomas Lim <Thomas.Lim@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>