drm/xe/lrc: Remove leftover TODO/FIXME

There isn't anything to set for CTX_TIMESTAMP handling in the empty
LRC: that is set on every LRC init since it should always start from 0
rather than the value saved in the image after first submission.

The FIXME about perma-pinning also doesn't make much sense as we will
always going to pin the lrc and the GGTT mapping has nothing to do with
VM bind.

Nuke these leftover comments.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250710-lrc-refactors-v2-5-a5e2ca03f6bd@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>