i965: Avoid redundant call to brw_merge_inputs() in brw_try_draw_prims()

i965: Avoid redundant call to brw_merge_inputs() in brw_try_draw_prims()

We always call brw_merge_inputs() right before looping over the primitives but
this can be called inside the loop for each primitive too. In the case we do it
for the first primitive the call is redundant and can be skipped.

Reviewed-by: Eric Anholt <eric@anholt.net>