libdrm/amdgpu: Use private fd for amdgpu_device and winsys hash table to fix ZaphodHeads. (v2)

The amdgpu_device for a device node needs its own dup'ed fd, instead
of using the original fd passed in for a screen, to make multi-x-screen
ZaphodHeads configurations work on amdgpu.

The original fd's lifetime differs from that of the amdgpu_device, and from the
one stored in the hash. The hash key is the fd, and in order to compare hash
entries we fstat them, so the fd must be around for as long as the amdgpu_device
is.

This patch for libdrm/amdgpu is a translation of the radeon-winsys ZaphodHeads
fix for mesa's radeon-winsys, from mesa commit 28dda47ae4d974e3e032d60e8e0965c8c068c6d8

"winsys/radeon: Use dup fd as key in drm-winsys hash table to fix ZaphodHeads."

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>

v2: Check for valid fd's being >= 0, because fd == 0 is in theory
    a valid, although unlikely, fd and fd == -1 would denote an
    invalid fd. Thanks to William Lewis for pointing this out.

Reported-by: William Lewis <minutemaidpark@hotmail.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>