drm/i915/guc: Dump error capture to dmesg on CTB error

In the past, There have been sporadic CTB failures which proved hard
to reproduce manually. The most effective solution was to dump the GuC
log at the point of failure and let the CI system do the repro. It is
preferable not to dump the GuC log via dmesg for all issues as it is
not always necessary and is not helpful for end users. But rather than
trying to re-invent the code to do this each time it is wanted, commit
the code but for DEBUG_GUC builds only.

v2: Use IS_ENABLED for testing config options.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230418181744.3251240-3-John.C.Harrison@Intel.com