etnaviv: Fix too early gem unmap

With !233 (closed) we can unmap BOs too early. This of mitigated by !249 (merged) and can happen on external screen unplug (there are other scenarios but this is the most likely one). I have that on my todo list but there wasn't a bug so far. Trase looks somewhat like

[  488.735126] Call trace:
[  488.735132]  etnaviv_gem_close_object+0xc8/0xd0
[  488.735136]  drm_gem_object_release_handle+0x40/0xa8
[  488.735140]  drm_gem_handle_delete+0x70/0xc8
[  488.735144]  drm_gem_close_ioctl+0x38/0x50
[  488.735148]  drm_ioctl_kernel+0xcc/0x120
[  488.735151]  drm_ioctl+0x250/0x4a0