drm/i915: load boot context at driver init time

According to the bspec for MBCTL:

Driver must set bit in the following scenarios:
  - to realod teh h/w boot context every time it gets loaded through OS
  - after an FLR clears the register (BIOS won't run afterwards)

References: https://bugs.freedesktop.org/show_bug.cgi?id=50237
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>