radeon/winsys: keep screen pointer in winsys v2

Only create one screen for each winsys instance.
This helps with buffer sharing and interop handling.

v2: rebased and some minor cleanup

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
8 files changed