drm/amd/display: Fix NULL deref and buffer over-read in SDP debugfs

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix NULL deref and buffer over-read in SDP debugfs [Why & How] dp_sdp_message_debugfs_write() dereferences connector->base.state->crtc without checking for NULL. A connector can be connected but not bound to any CRTC (e.g. after hot-plug before the next atomic commit), causing a kernel crash when writing to the sdp_message debugfs node. The function also ignores the user-provided size argument and always passes 36 bytes to copy_from_user(), reading past the user buffer when size < 36. Fix both issues by: - Returning -ENODEV when connector->base.state or state->crtc is NULL - Clamping write_size to min(size, sizeof(data)) (cherry picked from commit 6ab4c36a522842ff70474a1c0af2e40e50fc8300)