mesa: glx: Reduce error log level

mesa: glx: Reduce error log level

The code that parses LIBGL_DRIVERS_PATH was printing an
error for every attempted dlopen. It's not an error to
have to check multiple items in the path, only an error if
no suitable library is found. Reduced the load error to
a warning to match behavior of dynamic linker.

Signed-off-by: Courtney Goeltzenleuchter <courtney@LunarG.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>