v3d: do not emit thrsw between tmud and tmua

@anholt: there are some shaders in piglit that hit this scenario but it doesn't look like it was causing any problems. The simulator doesn't complain about this either, however, the V3D 4.2 spec states that this is not allowed, so I figured we might want to play it safe, just in case. What do you think? I am not sure what policy you have been following regarding cases like this in the past.

Shader db impact:

total instructions in shared programs: 9102719 -> 9136359 (0.37%)
instructions in affected programs: 239195 -> 272835 (14.06%)
helped: 0
HURT: 368