drm/xe/uapi: Move xe_exec after xe_exec_queue

Although the exec ioctl is a very important one, it makes no sense
to explain xe_exec before explaining the exec_queue. So, let's
move this down to help bring a better flow on the documentation
and code readability.

It is important to highlight that this patch is changing all
the ioctl numbers in a non-backward compatible way. However, we
are doing this final uapi clean-up before we submit our first
pull-request to be part of the upstream Kernel. Once we get
there, no other change like this will ever happen and all the
backward compatibility will be respected.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>