CVE-2026-68235: drm/amd/display: dce100: skip non-DP stream encoders for DP MST

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

drm/amd/display: dce100: skip non-DP stream encoders for DP MST

On DCE8-class ASICs (e.g. Bonaire), the resource pool contains digital DIG stream encoders plus one analog DAC encoder. When assigning a stream encoder for a second DisplayPort MST stream, if the preferred digital encoder is already acquired, dce100findfirstfreematchstreamencforlink() falls back to the first free pool entry. That entry may be the analog encoder, whose funcs table lacks DP hooks such as dpsetstreamattribute. The subsequent atomic commit then dereferences NULL function pointers in linksetdpmson() and crashes.

Skip encoders without dpsetstreamattribute when the stream uses a DP signal (including MST). Use dcisdpsignal(stream->signal) for the MST fallback path instead of checking only the link connector signal.

Tested on: - GPU: AMD Radeon R7 260X (Bonaire / DCE8) - Board: Supermicro C9X299-PG300 - Setup: DP MST daisy chain, hotplug second monitor or have it connected on boot - Kernel: 7.1.3 (issue observed since 6.19) - Result: kernel oops without patch; dual monitors stable with patch

(cherry picked from commit 28ec64943e3ee4d9b8d30cea61e380f1429953a8)