fix VkDrmFormatModifierPropertiesListEXT encoding/decoding

The diff to the driver is

-    /* skip val->drmFormatModifierCount */
+    vn_encode_uint32_t(enc, &val->drmFormatModifierCount);

and the diff to the renderer is

-    /* skip val->drmFormatModifierCount */
+    vn_decode_uint32_t(dec, &val->drmFormatModifierCount);

They break the protocol. Using a driver with an incompatible virglrenderer will result in

• vkCreateInstance failing with VK_ERROR_INITIALIZATION_FAILED
• VN_DEBUG=init logging wire format version 0 != 100000

Marking as draft and see if I can work around it somehow.

/cc @ryanneph @zzyiwei