i965: Allocate at least some URB space even when max_vertices = 0.

Allocating zero URB space is a really bad idea.  The hardware has to
give threads a handle to their URB space, and threads have to use that
to terminate the thread.  Having it be an empty region just breaks a
lot of assumptions.  Hence, why we asserted that it isn't possible.

Unfortunately, it /is/ possible prior to Gen8, if max_vertices = 0.
In theory a geometry shader could do SSBO/image access and maybe
still accomplish something.  In reality, this is tripped up by
conformance tests.

Gen8+ already avoids this problem by placing the vertex count DWord
in the URB entry header.  This fixes things on earlier generations.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit a41f5dcb141a11ca5ca0c765c305027b0f0b609e)

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