meta: Use sampler object for mipmap generation

meta: Use sampler object for mipmap generation

Sampler objects are perfect for meta operations.Sampler object
is separate state object that shadows the sampling state in texture
object. With sampler object mipmap can maintain same sampling state for
all subsequent generation requests.

Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>