drm: Remove superflous linux/fb.h includes

Everyone who uses the fbdev emulation helpers doesn't need to include
fb.h directly. Remove it.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470847958-28465-3-git-send-email-daniel.vetter@ffwll.ch