drm/xe: Fix unreffed ptr leak on engine lookup

drm/xe: Fix unreffed ptr leak on engine lookup

The engine xarray holds a ref to engine, guarded by the lock.
While we do lookup for engine, we need to take the ref inside
the lock to prevent unreffed pointer escaping and
causing potential use-after-free after.

v2: remove branch prediction hint (Thomas)

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230602172732.1001057-1-mika.kuoppala@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>