ir3: fix calling ir3_const_state_mut in the binning VS
lower_driver_param_to_ubo would call ir3_const_state_mut unconditionally. However, since 850f2aab ("ir3, tu: Use a UBO for VS primitive params on a750+"), it can be called for the binning VS, causing an assert. This commit makes sure to only call ir3_const_state_mut when it's really necessary to have mutable access to the const state.