vk/device: Be consistent about path to DRM device

Function fill_physical_device() has a 'path' parameter, and struct
anv_physical_device has a 'path' member. Sometimes these are used;
sometimes hardcoded "/dev/dri/renderD128" is used instead.

Be consistent. Hardcode "/dev/dri/renderD128" in exactly one location,
during initialization of the physical device.
diff --git a/src/vulkan/device.c b/src/vulkan/device.c
index 61e29a7..ca8fd84 100644
--- a/src/vulkan/device.c
+++ b/src/vulkan/device.c
@@ -47,7 +47,7 @@
 {
    int fd;
    
-   fd = open("/dev/dri/renderD128", O_RDWR | O_CLOEXEC);
+   fd = open(path, O_RDWR | O_CLOEXEC);
    if (fd < 0)
       return vk_error(VK_ERROR_UNAVAILABLE);
 
@@ -377,7 +377,7 @@
    parse_debug_flags(device);
 
    device->instance = physicalDevice->instance;
-   device->fd = open("/dev/dri/renderD128", O_RDWR | O_CLOEXEC);
+   device->fd = open(physicalDevice->path, O_RDWR | O_CLOEXEC);
    if (device->fd == -1)
       goto fail_device;