filter: remove an unnecessary layer of indirection

filter: remove an unnecessary layer of indirection

This is a leftover from when some of the filter code was shared between
pointer acceleration methods (pre v1.11 or so). Now these functions are
duplicated across files, so both the names and what they do isn't
necessarily reflective anymore.

Let's drop one layer of indirection to make the code a bit easier to
understand.

No functional changes.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>