drm/xe/uapi: Fix documentation indentation

drm/xe/uapi: Fix documentation indentation

Fix these issues:

Documentation/gpu/driver-uapi:29: include/uapi/drm/xe_drm.h:817: WARNING:
+Bullet list ends without a blank line; unexpected unindent.
Documentation/gpu/driver-uapi:29: include/uapi/drm/xe_drm.h:835: WARNING:
+Definition list ends without a blank line; unexpected unindent.

Fixes: 75d37750a753 ("drm/xe/mmap: Add mmap support for PCI memory barrier")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/intel-xe/20250117164023.3fdc00b9@canb.auug.org.au/
Cc: Tejas Upadhyay <tejas.upadhyay@intel.com>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117193827.91779-1-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>