drm/i915: only hook up hpd pulse for DP outputs

On HSW+, the digital encoders are shared between HDMI and DP outputs,
with one encoder masquerading as both. The VBT should tell us if we need
to have DP or HDMI support on a particular port, but if we don't have DP
support and we enable the DP hpd pulse handler then we cause an oops.

Don't hook up the DP hpd handling if we don't have a DP port.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81856


Reported-by: Intel QA Team.
Signed-off-by: Dave Airlie <airlied@redhat.com> # v1
[ickle: Fix the error handling after a malloc failure]
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>