i965: Generalize intel_upload.c to support multiple uploaders.

i965: Generalize intel_upload.c to support multiple uploaders.

I'd like to reuse the upload logic for a new program cache, but the
buffers will need to have a different lifetime than the default
uploader, and also some address space restrictions.  So, we can't
use a single uploader for both situations - we'll need two of them.

This creates a public 'uploader' structure, and adjusts the interface
to take an uploader rather than always using brw->upload.  It should
have no functional change at the moment.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>