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

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

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

This commit addresses a null pointer dereference issue in the

`dcn20_program_pipe` function. 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.

Reported by smatch:

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn20/dcn20_hwseq.c:1925 dcn20_program_pipe() error: we previously assumed 'pipe_ctx->plane_state' could be null (see line 1877)