drm/amd/display: Correct the defined value for AMDGPU_DMUB_NOTIFICATION_MAX

[Why & How]
It actually exposes '6' types in enum dmub_notification_type. Not 5. Using smaller
number to create array dmub_callback & dmub_thread_offload has potential to access
item out of array bound. Fix it.

Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>