add i965 primitive restart hardware support where possible

v2:
 * Only enable NV_primitive_restart for gen >= 4
 * Move function names to the beginning of lines
 * Don't include brw_context.h from brw_draw.h
 * brw_handle_primitive_restart returns GLboolean instead
   of bool
 * Only call vbo_sw_primitive_restart once in
   brw_handle_primitive_restart
 * Add comments to various functions

v1:
 * Enable NV_primitive_restart for all hardware. (Software primitive
   restart is used where necessary)
 * If hardware supports cut index and cut index can handle the
   primitive restart scenario, then use cut index to implement
   hardware support for primitive restart during the draw.