drm/xe/lrc: Use a temporary buffer for WA BB

drm/xe/lrc: Use a temporary buffer for WA BB

In case the BO is in iomem, we can't simply take the vaddr and write to
it. Instead, prepare a separate buffer that is later copied into io
memory. Right now it's just a few words that could be using
xe_map_write32(), but the intention is to grow the WA BB for other
uses.

Fixes: 82b98cadb01f ("drm/xe: Add WA BB to capture active context utilization")
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://lore.kernel.org/r/20250604-wa-bb-fix-v1-1-0dfc5dafcef0@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>