loader_dri3/glx/egl: Reinstate the loader_dri3_vtable get_dri_screen callback

loader_dri3/glx/egl: Reinstate the loader_dri3_vtable get_dri_screen callback

Removing this callback caused rendering corruption in some multi-screen cases,
so it is reinstated but without the drawable argument which was never used
by implementations and was confusing since the drawable could have been
created with another screen.

Cc: "17.3 18.0" mesa-stable@lists.freedesktop.org
Fixes: 5198e48a0d (loader_dri3/glx/egl: Remove the loader_dri3_vtable get_dri_screen callback)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105013
Reported-by: Daniel van Vugt <daniel.van.vugt@canonical.com>
Tested-by: Timo Aaltonen <tjaalton@ubuntu.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>