vt: export RGB offsets with FBIO_GETRGBOFFS
Add a new ioctl to vt to make it possible to export RGB offsets
set by vt drivers. This is needed to fix colors on X and Mesa
on some machines, especially on modern PowerPC64 BE ones.
With the appropriate changes in SCFB, to use this ioctl to find
out the correct RGB offsets, this fixes wrong colors on Talos II
and Blackbird, when used with their built-in video cards.
Reviewed by: alfredo
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D29000