drm: Move plane helpers into drm_kms_helper.ko

The drm core shouldn't depend upon any helpers, and we make sure this
doesn't accidentally happen by moving them into the helper-only
drm_kms_helper.ko module.

v2: Don't break the build for vmwgfx, spotted by Matt.

v3: Unbreak the depency loop around CONFIG_FB (not actually a loop
since it involves select). Reported by Chris.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>