drm/i915: add i2c symlink under hdmi connector

Currently, the i2c adapter is available only under DP connectors.

Add i2c symlink under hdmi connector pointing to i2c adapter in order to
make this behaviour consistent.

The initial motivation was to make igt i2c subtest
patch [1] work on all connectors.

[1]: https://patchwork.freedesktop.org/series/60357/



v2:
- Moved symlink remove to unregister (Ville)
- Clarified commit message (Jani)
- Changed WARN to DRM_ERROR (Jani)
- Minor codestyle changes proposed by Jani

v3: added blank line

Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190520150642.3477-1-oleg.vasilev@intel.com