drm/i915: Early creation of relay channel for capturing boot time logs

drm/i915: Early creation of relay channel for capturing boot time logs

As per the current i915 Driver load sequence, debugfs registration is done
at the end and so the relay channel debugfs file is also created after that
but the GuC firmware is loaded much earlier in the sequence.
As a result Driver could miss capturing the boot-time logs of GuC firmware
if there are flush interrupts from the GuC side.
Relay has a provision to support early logging where initially only relay
channel can be created, to have buffers for storing logs, and later on
channel can be associated with a debugfs file at appropriate time.
Have availed that, which allows Driver to capture boot time logs also,
which can be collected once Userspace comes up.

v2:
- Remove the couple of FIXMEs, as now the relay channel will be created
  early before enabling the flush interrupts, so no possibility of relay
  channel pointer being modified & read at the same time from 2 different
  execution contexts.
- Rebase.

v3:
- Add a comment to justiy setting 'is_global' before the NULL check on the
  parent directory dentry pointer.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>