freedreno/a6xx: refactor fd6_tex_swiz()

freedreno/a6xx: refactor fd6_tex_swiz()

We need a version of fd6_tex_swiz() that just returns the composed
swizzle without building part of the TEX_CONST_0 state.  So just
refactor the existing function to build more of the TEX_CONST_0 state,
and leave fd6_tex_swiz() simply composing swizzles.

The small IBO state change (to use LINEAR for smaller sizes/levels) is
to match the state in fd6_tex_const_0().  It seems like maybe tiled
actually works at the smaller sizes but not if minification is in play,
so best just to make images match what we do for textures.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>