drm/kms: slave encoder interface.

Define some helper functions to make easier to detach a KMS encoder
implementation from the drm module of the GPU it's used in. This is
mainly useful for some external I2C encoders known to be present on
cards with GPUs from several different manufacturers.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>