drm/amd/display: Decrease messaging about DP alt mode state to debug

Currently plugging in a USB-C device that issues an HPD will emit
a warning level message `DP Alt mode state on HPD: %d`.

This is needlessly noisy for most people, decrease it to debug so
that it can be turned on by dynamic debug as needed.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>