drm/xe/lrc: Add helper to capture context timestamp

Add a helper to capture CTX_TIMESTAMP from the context image so it can
be used to calculate the runtime.

v2: Add kernel-doc to clarify expectation from caller

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517204310.88854-4-lucas.demarchi@intel.com


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