drm/i915/guc: Speed up GuC log dumps

Add support for telling the debugfs interface the size of the GuC log
dump in advance. Without that, the underlying framework keeps calling
the 'show' function with larger and larger buffer allocations until it
fits. That means reading the log from graphics memory many times - 16
times with the full 18MB log size.

v2: Don't return error codes from size query. Report overflow in the
error dump as well (review feedback from Daniele).

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211211065859.2248188-2-John.C.Harrison@Intel.com