st/mesa: avoid sw fallback for getting/decompressing textures

st/mesa: avoid sw fallback for getting/decompressing textures

If st_GetTexImage() is to decompress the texture, avoid the fallback
path even if prefer_blit_based_texture_transfer = false.  For drivers
that returned PIPE_CAP_PREFER_BLIT_BASED_TEXTURE_TRANSFER = 0, we
were always taking the fallback path for texture decompression rather
than rendering a quad.  The later is a lot faster.

Cc: "10.0" "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>