gallium: remove library_path argument from pipe_loader_create_screen()

gallium: remove library_path argument from pipe_loader_create_screen()

Currently the location is determined at configure/build time and
consistently copied across gallium. Just remove the extra argument, and
use PIPE_SEARCH_DIR where appropriate.

This will allow us to remove the duplication in the *configuration and
*screen_create APIs by moving util_dl_get_proc_address() and friends to
probe time.

v2: rebase on top of vl_winsys_drm.c addition

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robclark@freedesktop.org>