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 8e4ed3cf ("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.

Fixes: 8e4ed3cf ("drm/amd/display: Add null check for pipe_ctx->plane_state in dcn20_program_pipe")
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>