i965: Add brw_prog_key_set_id helper to set the program id on any stage

i965: Add brw_prog_key_set_id helper to set the program id on any stage

For saving programs (shader cache; get program binary) it is useful to
set the id to 0, with the stage being a parameter.

For restoring programs it is useful to set the id to the id allocated
to the program at creation time.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>