drm/xe: Drop __engine_mask

Not really used, it's just a copy of engine_mask, which already reads
the fuses to mark engines as available/not-available.

Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240513213751.1017791-1-lucas.demarchi@intel.com


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