drm/v3d: Don't try to set OVRTMUOUT on V3D 4.x.

The old field is gone and the register now has a different field,
QRMAXCNT for how many TMU requests get serviced before thread switch.
We were accidentally reducing it from its default of 0x3 (4 requests)
to 0x0 (1).

v2: Skip setting the reg at all on 4.x, instead of trying to update
    only the old field.

Signed-off-by: Eric Anholt <eric@anholt.net>