gallium: add basic support for BPTC formats

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
mesa/st: expose ARB_texture_compression_bptc if BPTC formats supported

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50,nvc0: add BPTC format support

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>