drm/i915: clean up shrinker_release_pages

Add some proper flags for the different modes, and shorten the name to
something more snappy.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215110746.865-2-matthew.auld@intel.com