mesa/st: ETC2 now uses R8G8B8A8_SRGB as fallback

The check for ETC2 compatibility was not updated when the fallback
format was changed.

Fixes: 71867a0a
   st/mesa: Fall back to R8G8B8A8_SRGB for ETC2

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit e94095ec)