drm/v3d: Drop unused v3d_flush_caches().

Now that I've specified how the end-of-pipeline flushing should work,
we're never going to use this function.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Dave Emett <david.emett@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-2-eric@anholt.net