gallium: add PIPE_CAP_ASTC_DECODE_MODE and expose extension

mesa will expose GL_EXT_texture_compression_astc_decode_mode
extension if the cap is enabled by the driver.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <!30560>