v3d: use inc/dec tmu operation with image atomic sub/add of 1

v3d: use inc/dec tmu operation with image atomic sub/add of 1

This allows to remove a mov of 1/-1, as it is implicit with the
operation.

As with atomic inc/dec/add, usual shader-db set doesn't include any
GLES shader using it. So using as workaround vk-gl-cts shaders, we get
this:

total instructions in shared programs: 1217013 -> 1217006 (<.01%)
instructions in affected programs: 53 -> 46 (-13.21%)
helped: 2
HURT: 0

One of the helped shader went from 40 to 34 instructions.

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