drm/i915/gem: Make i915_gem_shrinker multi-gt aware

Where applicable, use for_each_gt instead of to_gt in the
i915_gem_shrinker functions to make them apply to more than just the
primary GT.  Specifically, this ensure i915_gem_shrink_all retires all
requests across all GTs, and this makes i915_gem_shrinker_vmap unmap
VMAs from all GTs.

v2: Pass correct GT to intel_gt_retire_requests(Andrzej).
v3: Remove unnecessary braces(Andi)
v4: Undo v3 to fix build failure.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926093028.23614-1-nirmoy.das@intel.com