drm/i915: Rework drop caches for checkpatch

With an upcoming change to bind, to make checkpatch happy and keep the
code clean, we need to rework this code a bit.

This should have no functional impact.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add the newline Chris requested.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>