drm/i915/bios: move i915_vbt debugfs to intel_bios.c

All things VBT should be placed in intel_bios.c.

While at it, shove in a FIXME comment about VBT possibly originating
from other places than opregion.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5e14a9559b6916022b506e5eb8d943783dc627a2.1704992868.git.jani.nikula@intel.com