freedreno/ir3: Track whether shader needs derivatives

freedreno/ir3: Track whether shader needs derivatives

In 1088b788 ("freedreno/ir3: find # of samplers from uniform vars") we
started counting number of samplers based on the uniform vars instead
of number of cat5 instructions.  We used the number of samplers to
determine whether to enable derivatives, but when we only use
derivatives and no samplers, that now breaks.  Track whether we need
derivatives explicitly and use that to enable the state.

Fixes: 1088b788 ("freedreno/ir3: find # of samplers from uniform vars")
Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>