drm: Add separate Kconfig option for fbdev helpers

For drivers which might want to disable fbdev legacy support.

Select the new option in all drivers for now, so this shouldn't result
in any change. Drivers need some work anyway to make fbdev support
optional (if they have it implemented, that is), so the recommended
way to expose this is by adding per-driver options. At least as long
as most drivers don't support disabling the fbdev support.

v2: Update for new drm drivers msm and rcar-du. Note that Rob's msm
driver can already take advantage of this, which allows us to build
msm without any fbdev depencies in the kernel!

v3: Move the MODULE_* stuff from the fbdev helper file to
drm_crtc_helper.c.

Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Acked-by: Dave Airlie <airlied@linux.ie>
Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>