intel: annotate the private symbols

They are less and easier to track than the public ones. The macro
drm_public will be going away by the end of the series.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>