The CMA helper now has the functionality to ensure a virtual address on
imported buffer so use that.

While touching all tinydrm drivers, remove the unnecessary inclusion of
drm_fb_helper.h in some drivers.

Cc: David Lechner <david@lechnology.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110145647.17580-6-noralf@tronnes.org