drm/v3d: Stop trying to flush L2C on V3D 3.3+

This cache was replaced with the slice accessing the L2T in the newer
generations.  Noted by Dave during review.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-5-eric@anholt.net


Reviewed-by: Dave Emett <david.emett@broadcom.com>