st/mesa: Allow ASTC5x5 fallbacks separately from other ASTC LDR formats.

st/mesa: Allow ASTC5x5 fallbacks separately from other ASTC LDR formats.

This patch allows us to fake ASTC 5x5 specifically, while leaving the
other ASTC LDR formats with native support.  I plan to use this in iris,
at least for the time being.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>