drm/amdgpu: Fix the warning division or modulo by zero

Checks the partition mode and returns an error for an invalid mode.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>