drm/i915: Run relevant bits of debugfs drop_caches per GT

Walk all GTs when doing the respective bits of drop_caches work.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907125808.186088-1-andi.shyti@linux.intel.com