i965: Make emit_urb_writes() reserve space for GS header information.

i965: Make emit_urb_writes() reserve space for GS header information.

Geometry shaders have additional header data at the beginning of their
output URB entries.  Shaders that use EndPrimitive() or multiple streams
have a control data header; shaders with a dynamic vertex count have an
additional vec4 slot to hold the 32-bit vertex count (and 96 bits of
padding).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>