i965: Introduce brw_prepare_drawing.

i965: Introduce brw_prepare_drawing.

In order to add our ARB_indirect_parameters implementation we
need to refactor brw_try_draw_prims so that it operates on a
per primitive basis and move the loop into brw_draw_prims.
This commit introduces the brw_prepare_drawing function where
we move the code that executes once before the loop.

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>