mesa: Make ARB_sampler_objects mandatory

mesa: Make ARB_sampler_objects mandatory

To allow meta acceleration operations to use sampler objects the
ARB_sampler_objects extension needs to be mandatory for all drivers.
Because the extension doesn't have any hardware dependencies it is
trivial to implement.

Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>