r600: factor out the code to initialise a buffer resource.

r600: factor out the code to initialise a buffer resource.

This takes the code required to initialise a buffer resource
out of the texture buffer code, into it's own function.

This is going to be used for the image support later.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>