Remove the unused DRM_DISPLAY_INFO_LEN from the uapi headers.
I presume the original plan was to expose the display name
via getconnector, but looks like that never happened. So we have
the define for the length of the string but no string anywhere.

A quick scan didn't seem to reveal userspace referencing this
so hopefully we can just nuke it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326173401.7329-4-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>