i965: Resolve GCC sign-compare warning.

i965: Resolve GCC sign-compare warning.

mesa/src/mesa/drivers/dri/i965/brw_draw_upload.c: In function 'brw_prepare_vertices':
mesa/src/mesa/drivers/dri/i965/brw_draw_upload.c:434:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = j = 0; i < brw->vb.nr_enabled; i++) {
                      ^
mesa/src/mesa/drivers/dri/i965/brw_draw_upload.c:557:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < nr_uploads; i++) {
                 ^
mesa/src/mesa/drivers/dri/i965/brw_draw_upload.c:569:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < nr_uploads; i++) {
                  ^

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