i965: Always use BRW_TEXCOORDMODE_CUBE when seamless filtering.

i965: Always use BRW_TEXCOORDMODE_CUBE when seamless filtering.

When using seamless cube map mode and NEAREST filtering, we explicitly
overrode the wrap modes to CLAMP_TO_EDGE.  This was to implement the
following spec text:

   "If NEAREST filtering is done within a miplevel, always apply apply
    wrap mode CLAMP_TO_EDGE."

However, textureGather() ignores the sampler's filtering mode, and
instead returns the four pixels that would be blended by LINEAR
filtering.  This implies that we should do proper seamless filtering,
and include pixels from adjacent cube faces.

It turns out that we can simply delete the NEAREST -> CLAMP_TO_EDGE
overrides.  Normal cube map sampling works by first selecting the
face, and then nearest filtering fetches the closest texel.  If the
nearest texel was on a different face, then that face would have been
chosen.  So it should always be within the face anyway, which
effectively performs CLAMP_TO_EDGE.

Fixes 86 dEQP-GLES31.texture.gather.basic.cube.* tests.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Suggested-by: Ian Romanick <idr@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>