intel: Use a URB start offset of 0 for disabled stages.

intel: Use a URB start offset of 0 for disabled stages.

There are some cases where the VS is the only stage enabled, it uses the
entire URB, and the URB is large enough that placing later stages after
the VS exceeds the number of bits for "URB Starting Address".

For example, on Icelake GT2, "varying-packing-simple mat2x4 array" from
Piglit is getting a starting offset of 128 for the GS/HS/DS.  But the
field is only large enough to hold an offset of 127.

i965 doesn't hit any genxml assertions because it's still using the old
OUT_BATCH mechanism.  128 << GEN7_URB_STARTING_ADDRESS_SHIFT (57) == 0,
with the extra bit falling off the end.  So we place the disabled stage
at the beginning of the URB (overlapping with push constants).  This is
likely okay since it's a zero size region (0 entries).

It seems like the Vulkan driver might hit this assertion, however, and
the situation seems harmless.  To work around this, always place
disabled stages at the start of the URB, so the last enabled stage can
fill the remaining space without overflowing the field.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>