drm/amd/display: reset dcn31 SMU mailbox on failures

Otherwise future commands may fail as well leading to downstream
problems that look like they stemmed from a timeout the first time
but really didn't.

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