drm/xe: Save CTX_TIMESTAMP mmio value instead of LRC value

For determining actual job execution time, save the current value of the
CTX_TIMESTAMP register rather than the value saved in LRC since the
current register value is the closest to the start time of the job.

v2: Define MI_STORE_REGISTER_MEM to fix compile error
v3: Place MI_STORE_REGISTER_MEM sorted by MI_INSTR (Lucas)

Fixes: 65921374 ("drm/xe: Emit ctx timestamp copy in ring ops")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250509161159.2173069-6-umesh.nerlige.ramappa@intel.com