drm: rename u32 in __u32 in uapi

All other fields use __

Cc: Ben Widawsky <ben@bwidawsk.net>
Fixes: db1689aa ("drm: Create a format/modifier blob")
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170824150814.5878-1-lionel.g.landwerlin@intel.com