i965: Use the proper element of the prim array in brw_try_draw_prims.

i965: Use the proper element of the prim array in brw_try_draw_prims.

The VBO module actually calls us with an array of _mesa_prim objects.
For example, it may break up a DrawArrays() call into multiple
primitives when primitive restart is enabled.

Previously, we treated prim like a pointer, always accessing element 0.
This worked because all of the primitive objects in a single draw call
have the same value for num_instances and basevertex.

However, accessing an array as a pointer and using the wrong object's
fields is misleading.  For stylistic reasons alone, we should use the
right object.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>