drm/v3d: Don't bother flushing L1TD at job start.

This is the write combiner for TMU writes.  You're supposed to flush
that at job end if you had dirtied any cachelines.  Flushing it at job
start then doesn't make any sense.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Reviewed-by: Dave Emett <david.emett@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-3-eric@anholt.net
(cherry picked from commit 2e6dc3bd)