i965: Only use brw_draw.c's trim() function when necessary.

brw_draw.c contains a trim() function which modifies the vertex count
for quads and quad strips in order to discard dangling vertices.  In
principle this shouldn't be necessary, since hardware since Gen4 is
capable of discarding dangling vertices by itself.  However, it's
necessary because as a hack to speed up rendering on Gen 4-5, we
sometimes convert quads to trifans and quad strips to tristrips.  The
trim() function isn't necessary on Gen6 and up.

This patch documents why and when the trim() function is necessary,
and avoids calling it when it's not needed.

This will avoid creating problems when we enable hardware support for
primitive restart of quads and quad strips on Haswell.