drm: Move port definition back to i915 header

We dont need the definition of the enum port outside I915, anymore.
Hence move enum port definition into I915 driver itself.

v2:
  intel_display.h is included in intel_hdcp.h
v3:
  enum port is declared in headers.
v4:
  commit msg is rephrased.
v5:
  copyright year is updated [Tomas]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190828164216.405-3-ramalingam.c@intel.com