vulkan/runtime: Allow more than 8 DRM devices

Some people seem to have systems with more than 8 GPUs installed at
once. 256 is the maximum number of devices returned by libdrm, so using
this seems like a good choice for now.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27901>

src/vulkan/runtime/vk_instance.c

@@ -389,8 +389,8 @@ vk_instance_add_driver_trace_modes(struct vk_instance *instance,
 static VkResult
 enumerate_drm_physical_devices_locked(struct vk_instance *instance)
 {
-   /* TODO: Check for more devices ? */
-   drmDevicePtr devices[8];
+   /* libdrm returns a maximum of 256 devices (see MAX_DRM_NODES in libdrm) */
+   drmDevicePtr devices[256];
    int max_devices = drmGetDevices2(0, devices, ARRAY_SIZE(devices));
 
    if (max_devices < 1)