st/mesa: Make transcode_astc also check for non-SRGB format support

st/mesa: Make transcode_astc also check for non-SRGB format support

This is probably unnecessary in that all drivers which support the sRGB
format likely also support the non-sRGB format.  But we may as well
check both the formats we use, for documentation if nothing else.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15875>