i965/blorp/exec: Refactor to use a new blorp_batch struct

i965/blorp/exec: Refactor to use a new blorp_batch struct

This gets rid of brw_context throughout the core of the state setup code.
Instead, it is replaced with blorp_batch which contains a pointer to the
blorp_context and a void* that the driver can use for its own blorp data.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>