drm/i915/selftests: Consider multi-gt instead of to_gt()

In order to enable complete multi-GT, loop through all
the GTs, rather than relying on the to_gt(), which only
provides a reference to the primary GT.

Problem appear when it runs on platform like MTL where
different set of engines are possible on different GTs.

Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230419060036.3422635-4-tejas.upadhyay@intel.com