drm/amdgpu/virt: increase mailbox timeout to 5000ms

When multiple VFs try to enter exclusive mode at the same time, the
looping mechansim doesn't help to ensure each can get it because it
only loops active VFs, then the last one has to wait for a long
interval.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Reviewed-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>