drm/i915: enum port definition is moved into i915_drm.h

For the reusability of the enum port in other driver modules
(like mei_hdcp), enum port definition is moved from I915 local header
intel_display.h to drm/i915_drm.h

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: Fix subject prefix.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1550219730-17734-3-git-send-email-ramalingam.c@intel.com