i965: Resolve GCC sign-compare warning.

i965: Resolve GCC sign-compare warning.

mesa/src/mesa/drivers/dri/i965/brw_draw.c: In function 'brw_draw_destroy':
mesa/src/mesa/drivers/dri/i965/brw_draw.c:630:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < brw->vb.nr_buffers; i++) {
                  ^
mesa/src/mesa/drivers/dri/i965/brw_draw.c:636:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < brw->vb.nr_enabled; i++) {
                  ^

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>