intel: allow GPU re-enumeration

When apps see XGL_ERROR_DEVICE_LOST, they release everything and enumerate
GPUs again.  The call to intel_gpu_remove_all() was in
xglInitAndEnumerateGpus(), but it was missing in the new xglEnumerateGpus().
diff --git a/icd/intel/instance.c b/icd/intel/instance.c
index 79876e7..44cad85 100644
--- a/icd/intel/instance.c
+++ b/icd/intel/instance.c
@@ -139,6 +139,8 @@
     XGL_RESULT ret;
     uint32_t count;
 
+    intel_gpu_remove_all();
+
     if (!maxGpus) {
         *pGpuCount = 0;
         return XGL_SUCCESS;