drm/xe: Add helper to capture engine timestamp

Just like CTX_TIMESTAMP is used to calculate runtime, add a helper to
get the timestamp for the engine so it can be used to calculate the
"engine time" with the same unit as the runtime is recorded.

Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517204310.88854-5-lucas.demarchi@intel.com


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