i965: Enable AMD_seamless_cubemap_per_texture

i965: Enable AMD_seamless_cubemap_per_texture

The change is very small.  Do seamless filtering if either the context
enable is set or the sampler enable is set.

The AMD_seamless_cubemap_per_texture says:

    "If TEXTURE_CUBE_MAP_SEAMLESS_ARB is emabled (sic) globally or the
    value of the texture's TEXTURE_CUBE_MAP_SEAMLESS_ARB parameter is
    TRUE, seamless cube map sampling is enabled..."

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>