drm: Don't EXPORT drm_add/reset_display_info

Only used within drm.ko, no need to tempt drivers.

Cc: Keith Packard <keithp@keithp.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424142242.12093-1-daniel.vetter@ffwll.ch