loader: let compiler figure out the length of the string

Basically, turn comment into code

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/loader/loader.c

@@ -110,8 +110,7 @@ static char *loader_get_dri_config_device_id(void)
 
 static char *drm_construct_id_path_tag(drmDevicePtr device)
 {
-/* Length of "pci-xxxx_xx_xx_x\0" */
-#define PCI_ID_PATH_TAG_LENGTH 17
+#define PCI_ID_PATH_TAG_LENGTH sizeof("pci-xxxx_xx_xx_x")
    char *tag = NULL;
 
    if (device->bustype == DRM_BUS_PCI) {