[Why]
We need HPD IRQ notifications (RX, short pulse) to properly handle
DP MST for DPIA connections.

[How]
A null pointer exception currently occurs when these are received
so add a check to validate that we have a handler installed for
the notification.

Extend the HPD handler to also handle HPD IRQ (RX) since the logic is
the same.

Fixes: e27c41d5 ("drm/amd/display: Support for DMUB HPD interrupt handling")
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Reviewed-by: Jude Shih <shenshih@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>