i965: Keep track of whether LRI is allowed in the context struct.

i965: Keep track of whether LRI is allowed in the context struct.

This stores the result of can_do_pipelined_register_writes() in the
context struct so we can find out later whether LRI can be used to
program the L3 configuration.

v2:
 * Split change of gen check in can_do_pipelined_register_writes (jljusten)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg  <krh@bitplanet.net>