Fix native x86 option rom initialization

Fix native x86 option rom initialization

- Intel option roms want an initialized i8259 or they will
  throw an exception 6. This should be done in the southbridge
  code, but that is executed much later than the VGA init, so
  initialize the i8259 in src/devices/oprom/x86.c.
  In the long run this will allow getting rid of some of the
  ugly hacks in some AMD boards' romstage.c
- Don't overwrite the mode when copying mode info information back
  from 0x600.

Change-Id: Idb01f13dbcd736d8d830b222ffe1ea85799fcd9c
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/257
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>