drm/xe: Add explicit padding to uAPI definition

Pad the uAPI definition so that it would align identically between
64-bit and 32-bit uarch, so consumers using this header will work
correctly from 32-bit compat userspace on a 64-bit kernel. Do it
in a minimally invasive way, so that 64-bit userspace will still
work with the previous header, and so that no fields suddenly
change sizes.

Originally inspired by mlankhorst.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>