i965: Always try to create a logical context

i965: Always try to create a logical context

Always enable use of HW logical contexts to preserve GPU state between
batches when the kernel supports such constructs, continuing to enforce
the required support for gen6+.

At runtime, this effectively removes the BRW_NEW_CONTEXT flag (and the
upload of invariant state) from the start of every batch for any kernel
supporting contexts. So long as the older atoms are correctly listening
to the right flag (NEW_CONTEXT rather than NEW_BATCH) this should
eliminate a few redundant state uploads for the older platforms.

No piglits were harmed on ctg and ilk, both with and without logical
contexts.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>