drm/i915: Drop FBDEV #ifdev in mst code

Since

commit a03fdcb1
Author: Archit Taneja <architt@codeaurora.org>
Date:   Wed Aug 5 12:28:57 2015 +0530

    drm: Add top level Kconfig option for DRM fbdev emulation

this is properly handled using dummy functions. This essentially
undoes

commit 7296c849
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jul 22 20:10:28 2014 +1000

    drm/i915: fix build without fbde

v2: We also need to drop the #ifdef from headers. Seems like a small
price to pay for slightly cleaner code.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170704151833.17304-3-daniel.vetter@ffwll.ch