i965: Add hardware context support.

i965: Add hardware context support.

With fixes and updates from Ben Widawsky and comments from Paul Berry.

v2: Use drm_intel_gem_context_destroy to destroy hardware context;
    remove useless initialization of hw_ctx, both suggested by Eric.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Paul Berry <stereotype441@gmail.com>