drm/i915: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a return; statement:

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c:65:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>