radeonsi: fix texture gather on stencil textures

radeonsi: fix texture gather on stencil textures

At least on VI, texture gather doesn't work with a 24_8 data format, so
use 8_8_8_8 and a modified swizzle instead.

A bit of background: When creating a GL_STENCIL_INDEX8 texture, we select
the X24S8 pipe format because we don't support stencil-only render targets
properly. With mip-mapping this can lead to a setup where the tiling is
incompatible with stencil texturing, and a flushed stencil texture is
used. For the flushed stencil, a literal X24S8 is used because there were
issues with an 8bpp DB->CB copy.

Longer term, it would be good if we could get away from these workarounds,
i.e. properly support an S8 format for stencil-only rendering and flushed
stencil. Since stencil texturing is somewhat rare, it's not a high
priority.

Fixes GL45-CTS.texture_cube_map_array.sampling.

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>