drm/fb-helper: Add a dummy remove_conflicting_framebuffers

Lots of drivers don't properly compile without this when CONFIG_FB=n.
It's kinda a hack, but since CONFIG_FB doesn't stub any fucntions when
it's disabled I think it makes sense to add it to drm_fb_helper.h.

Long term we probably need to rethink all the logic to unload firmware
framebuffer drivers, at least if we want to be able to move away from
CONFIG_FB and fbcon.

v2: Unfortunately just stubbing out remove_conflicting_framebuffers in
drm_fb_helper.h upset gcc about static vs. non-static declarations, so
a new wrapper it needs to be. Means more churn :(

Cc: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: tomi.valkeinen@ti.com
Cc: dh.herrmann@gmail.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470847958-28465-2-git-send-email-daniel.vetter@ffwll.ch