drm/exynos: make fbdev support really optional

Currently enabling Exynos DRM support automatically pulls in
lots of fbdev dependencies. However these deps are
unnecessary since DRM core already enables them when
needed.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466769168-31602-1-git-send-email-tjakobi@math.uni-bielefeld.de