drm/i915: Allow error capture without a request

There was a report of error captures occurring without any hung
context being indicated despite the capture being initiated by a 'hung
context notification' from GuC. The problem was not reproducible.
However, it is possible to happen if the context in question has no
active requests. For example, if the hang was in the context switch
itself then the breadcrumb write would have occurred and the KMD would
see an idle context.

In the interests of attempting to provide as much information as
possible about a hang, it seems wise to include the engine info
regardless of whether a request was found or not. As opposed to just
prentending there was no hang at all.

So update the error capture code to always record engine information
if a context is given. Which means updating record_context() to take a
context instead of a request (which it only ever used to find the
context anyway). And split the request agnostic parts of
intel_engine_coredump_add_request() out into a seaprate function.

v2: Remove a duplicate 'if' statement (Umesh) and fix a put of a null
pointer.
v3: Tidy up request locking code flow (Tvrtko)
v4: Pull in improved info message from next patch and fix up potential
leak of GuC register state (Daniele)

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com> (v2)
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127002842.3169194-5-John.C.Harrison@Intel.com