drm/doc: include xe_drm.h

Make sure the uapi gets picked up by the normal docs build.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>