drm/i915/execlists: Ignore tracek for nop process_csb

Recording the frequent inspection of CSB head/tail when there is
expected to be no update adds noise to the debug trace. (Not entirely
useless, but since we know the sequence of function calls, we can
surmise the function was called -- so redundant.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200209131922.180287-2-chris@chris-wilson.co.uk