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.

Fixes: 0baae624 ("drm/amd/display: Refactor fast update to use new HWSS build sequence")
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>