drm/amd/display: decouple resource_pool from resource_context

to avoid null access in case res_ctx is used to access res_pool before it's fully constructed

also make it clear which function has dependency on resource_pool

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>