winsys/radeon: don't leak the fd when it is 0

winsys/radeon: don't leak the fd when it is 0

Earlier commit added an extra dup(fd) to fix a ZaphodHeads issue.
Although it did not consider the (very unlikely) case where we might end
up with the valid fd == 0.

Fixes: 28dda47ae4d(winsys/radeon: Use dup fd as key in drm-winsys hash
table to fix ZaphodHeads.)

Cc: 10.6 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>