loader: Handle failure to load DRI driver library

I factored out the chunk of loader code that dlopen()s libraries from the rest of the DRI driver loader function in this commit:

commit bc343154f816b637b200933e1c1a35db19f478ef
Author: James Jones <jajones@nvidia.com>
Date:   Thu Apr 22 23:17:08 2021 -0700

loader: Factor out driver library loading code

However, I failed to adjust the DRI loader function that now uses the new helper function to handle the case where the requested DRI library is not found.

This change restores the prior behavior, and also ensures loader_open_driver() consistently returns NULL in the out_driver_handle parameter on failure.

Fixes: bc343154 ("loader: Factor out driver library loading code") Signed-off-by: James Jones jajones@nvidia.com

Closes #5042 (closed)