drm/xe/uapi: Add missing documentation for struct members

This removes the documentation build warnings below:

	include/uapi/drm/xe_drm.h:828: warning: Function parameter or \
	member 'pad2' not described in 'drm_xe_vm_bind_op'
	include/uapi/drm/xe_drm.h:875: warning: Function parameter or \
	member 'pad2' not described in 'drm_xe_vm_bind'
	include/uapi/drm/xe_drm.h:1006: warning: Function parameter or \
	member 'handle' not described in 'drm_xe_sync'
	include/uapi/drm/xe_drm.h:1006: warning: Function parameter or \
	member 'timeline_value' not described in 'drm_xe_sync'
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

include/uapi/drm/xe_drm.h

@@ -816,7 +816,7 @@ struct drm_xe_vm_bind_op {
 	 */
 	__u32 prefetch_mem_region_instance;
 
-	/** @pad: MBZ */
+	/** @pad2: MBZ */
 	__u32 pad2;
 
 	/** @reserved: Reserved */
@@ -857,7 +857,7 @@ struct drm_xe_vm_bind {
 		__u64 vector_of_binds;
 	};
 
-	/** @pad: MBZ */
+	/** @pad2: MBZ */
 	__u32 pad2;
 
 	/** @num_syncs: amount of syncs to wait on */
@@ -982,6 +982,7 @@ struct drm_xe_sync {
 	__u32 flags;
 
 	union {
+		/** @handle: Handle for the object */
 		__u32 handle;
 
 		/**
@@ -995,6 +996,7 @@ struct drm_xe_sync {
 		__u64 addr;
 	};
 
+	/** @timeline_value: Timeline point of the sync object */
 	__u64 timeline_value;
 
 	/** @reserved: Reserved */