r600g: Manage fences per screen rather than per context.

r600g: Manage fences per screen rather than per context.

A fence is a screen object and can outlive the context it was created from.
The previous code would access freed memory in that case, resulting in
various problems.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=44151
       https://bugs.freedesktop.org/show_bug.cgi?id=44007

Probably fixes: https://bugs.freedesktop.org/show_bug.cgi?id=43993

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>