drm/i915: Check encoder type in enc_to_dig_port()

Don't allow conversion from arbitraty encoder types to a digital port.
Calling enc_to_dig_port() with the wrong encoder may seem far fetched,
but certain paths of the ddi code may be called with hasell's analog
encoder and the conversion is wrong for DP mst encoders too, so safe
guard against it.

v2: Warn if encoder type is unknown and device is not DDI. (Imre)
v3: Remove stray hunk from rebase error. (Ander)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224141845.5836-1-ander.conselvan.de.oliveira@intel.com