drm/amdgpu: rename rlc autoload to backdoor autoload

This is to differentiate rlc backdoor autoload from rlc
frontdoor autoload

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>