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>