vl/dri3: do full teardown on screen_destroy

Earlier commit added support for 'front_buffers', erroneously adding a
return in vl_dri3_screen_destroy. Effectively leaking a lot of state.

Fixes: 8d7ac0a4 ("vl/dri3: implement DRI3 BufferFromPixmap")
Cc: Leo Liu <leo.liu@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
(cherry picked from commit d081ad2a)