drm/xe/uapi: Make constant comments visible in kernel doc

As there is no direct way to make comments of constants directly
visible in the kernel doc, move them to the description of the
structure where they can be used. By doing so they appear in the
"Description" section of the struct documentation.

v2: Remove DRM_XE_UFENCE_WAIT_MASK_* (Francois Dugast)
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>