drm/xe: Drop duplicated declaration

The declaration of xe_reg_whitelist_process_engine() function does
not fit into "xe_wa.h" and is already a duplicate.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240613195702.2164-1-michal.wajdeczko@intel.com