drm/amd/display: Fix null check for pipe_ctx->plane_state in dcn20_program_pipe

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix null check for pipe_ctx->plane_state in dcn20_program_pipe This commit addresses a null pointer dereference issue in dcn20_program_pipe(). Previously, commit 8e4ed3cf1642 ("drm/amd/display: Add null check for pipe_ctx->plane_state in dcn20_program_pipe") partially fixed the null pointer dereference issue. However, in dcn20_update_dchubp_dpp(), the variable pipe_ctx is passed in, and plane_state is accessed again through pipe_ctx. Multiple if statements directly call attributes of plane_state, leading to potential null pointer dereference issues. This patch adds necessary null checks to ensure stability.