drm/amd/display: Fix null check for pipe_ctx->plane_state in resource_build_scaling_params

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix null check for pipe_ctx->plane_state in resource_build_scaling_params Null pointer dereference issue could occur when pipe_ctx->plane_state is null. The fix adds a check to ensure 'pipe_ctx->plane_state' is not null before accessing. This prevents a null pointer dereference. Found by code review. (cherry picked from commit 63e6a77ccf239337baa9b1e7787cde9fa0462092)