drm/i915/guc: Simplify GuC doorbell logic

All we're really doing is incrementing a simple counter in a
doorbell_info struct. We can do without extra variables and a separate
counter kept in guc_client. Since it's gone, we're also removing its
debugfs.
The only functional change here, is that we're no longer treating 0 as a
special value. GuC doesn't seem to care, why should we?

v2: Restore desc->tail update.
v3: Drop the retry loop, assert that doorbell cookie doesn't change
behind our back.
v4: WARN rather than BUG, use xchg. (Chris)

Cc: Chris Wilson <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Oscar Mateo <[email protected]>
Suggested-by: Chris Wilson <[email protected]>
Signed-off-by: Michał Winiarski <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>

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