freedreno/ir3: fix vertex shader sysvals with pre_assign_inputs

freedreno/ir3: fix vertex shader sysvals with pre_assign_inputs

The first pre_assign_inputs loop doesn't pre-assign sysvals, so skip the
second part for sysvals.

The sysvals don't need to be pre-assigned since the state for those isn't
shared between binning / nonbinning shaders.

Fixes assert failures in cases where the sysvals didn't end up in the same
registers for binning / nonbinning.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3168>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3168>