drm/xe: Add LRC ctx timestamp support functions

LRC ctx timestamp support functions are used to determine how long a job
has run on the hardware.

v2:
 - Don't use static inlines (Jani)
 - Kernel doc
 - s/ctx_timestamp_job/ctx_job_timestamp
v6:
 - Add kernel doc for xe_lrc_update_timestamp (Lucas)
 - Call xe_lrc_ctx_timestamp() in xe_lrc_update_timestamp (Lucas)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611144053.2805091-2-matthew.brost@intel.com