i965/gen6: Resolve GCC sign-compare warning.

i965/gen6: Resolve GCC sign-compare warning.

mesa/src/mesa/drivers/dri/i965/gen6_vs_state.c: In function 'gen6_upload_push_constants':
mesa/src/mesa/drivers/dri/i965/gen6_vs_state.c:85:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       for (i = 0; i < prog_data->nr_params; i++) {
                     ^
mesa/src/mesa/drivers/dri/i965/gen6_vs_state.c:92:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < prog_data->nr_params; i++) {
                 ^

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