drm/i915: Extend debugfs/i915_drop_caches to call i915_gem_shrink_all()

drm/i915: Extend debugfs/i915_drop_caches to call i915_gem_shrink_all()

Sometimes we want to explicitly page out all available objects from igt,
i.e. call i915_gem_shrink_all() and check that subsequent operations
succeed. This adds DROP_SHRINK_ALL [0x8] to the set of flags for
debugfs/i915_drop_caches for that purpose.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308144622.23194-1-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>