drm/amd/display: Remove unused code

This commit removes a legacy debug_defaults_diags struct.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>