drm/i915: Add initial_ prefix to bios fb takeover code

In spirit with

commit 5724dbd1
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Tue Jan 20 12:51:52 2015 +0000

    drm/i915: Rename plane_config to initial_plane_config

to make it clear that this code is all special-purpose for the initial
plane takeover.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>