drm/i915: check the return value of intel_dp_i2c_init

We've been ignoring this return value, so print a nice backtrace in
case it's not what we expected.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Zoltan Nyul <zoltan.nyul@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>