drm: fix compilations issues introduced by "drm: allow to use mmuless SoC"

Removing MMU configuration flag from DRM make few automatic
build failed when they answer yes to all flags.

Add asm/vga.h file on Blackfin architecture to not broke compilation.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Fixes: 62a0d98a ("drm: allow to use mmuless SoC")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483789151-6603-1-git-send-email-benjamin.gaignard@linaro.org