drm/i915/dp: Move initialization of hpd_pin to a new function

This shaves a few lines from intel_dp_init_connector() and will serve as
a good place to add other port specific information in a follow up
patch.

While at it, convert BUG() to MISSING_CASE() in the default case.

v2: s/BUG/MISSING_CASE. (Chris)
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170203140316.20792-2-ander.conselvan.de.oliveira@intel.com