drm/i915/guc: capture GuC logs if FW fails to load

We're currently deleting the GuC logs if the FW fails to load, but those
are still useful to understand why the loading failed. Keeping the
object around allows us to access them after driver load is completed.

v2: keep the object around instead of using kernel memory (chris)
    don't store the logs in the gpu_error struct (Chris)
    add a check on guc_log_level to avoid snapshotting empty logs

v3: use separate debugfs for error log (Chris)

v4: rebased

v5: clean up obj selection, move err_load inside guc_log, move err_load
    cleanup, rename functions (Michal)

v6: move obj back to intel_guc, move functions to intel_uc.c, don't
    clear obj on new GuC load, free object only if enable_guc_loading
    is set (Michal)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1495475428-19295-1-git-send-email-daniele.ceraolospurio@intel.com


Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Tested-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>