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

In the Linux kernel, the following vulnerability has been resolved:

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

This commit addresses a null pointer dereference issue in

hwss_setup_dpp(). The 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.