i965/sampler_state: Set the "Base Mip Level" field on Sandy Bridge

Fixes two GL ES 3.0 CTS tests on Sandy Bridge:

ES3-CTS.functional.texture.mipmap.cube.base_level.linear_linear
ES3-CTS.functional.texture.mipmap.cube.base_level.linear_nearest

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "17.0 13.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit c59d1ea51bd0809761094e54c66bf3a200d964ff)
2 files changed