drm/i915/i915_drv: Use proper parameter naming in for_each_engine()

for_each_engine() loops through engines in the GT, not in
dev_priv.  Because it's misleading, call it "gt__" instead of
"dev_priv__".

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230421190026.294208-2-andi.shyti@linux.intel.com