drm/amdgpu: rename fiji_smumgr.h to fiji_smum.h

This conflicts with fiji_smumgr.h from powerplay
in DKMS environment

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com>