drm/i915: always preserve bios swizzling

Currently we only set preserve_bios_swizzling when the initial fb is
shared and totally miss the single-screen case. Fix this by
consolidating all the logic for both cases.

This seems to go back to when swizzle preservation was originally
merged in

commit d9ceb816
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Oct 9 12:57:43 2014 -0700

    drm/i915: preserve swizzle settings if necessary v4

Cc: Kristian Høgsberg <hoegsberg@gmail.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>