i965: Use a WC map and memcpy for the batch instead of pwrite.

i965: Use a WC map and memcpy for the batch instead of pwrite.

We'd like to eliminate the malloc'd shadow copy eventually, but there
are still unresolved performance problems.  In the meantime, let's at
least get rid of pwrite.

On Apollolake, improves Synmark OglBatch6 performance by:
1.53581% +/- 0.269589% (n=108).

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>