i965/blorp: Reduce the size of vertex buffer

i965/blorp: Reduce the size of vertex buffer

Previously the vertex buffer consisted of eight floats per vertex
of which six where constants. These can be as easily provided by
vertex fetcher as it is capable of filling vertex elements with
constant one and zero. This reduces the size of the vertex buffer
from 3 * 8 * 4 = 96 to 3 * 2 * 4 = 24 bytes.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>