intel: annotate the intel genx helpers as private

They're used internally and never meant to be part of the API.
Add the drm_private notation, which should resolve that.

v2: (Rodrigo) Add missing include.

Cc: Eric Engestrom <eric.engestrom@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: 4e81d4f9 ("intel: add generic functions to check PCI ID")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>