winsys/radeon: Compute is_displayable in surf_drm_to_winsys

It was always 0, breaking (at least) DRI3 with Xwayland.

Bugzilla: https://bugs.freedesktop.org/104306


Fixes: 5f2073be ("ac/surface: add ac_surface::is_displayable")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 1cf1bf32)