drm/i915: Fix memory leak in intel_dsi_init() error path

intel_dsi_init() bails out without freeing the memory 'intel_dsi' and
'intel_connector' point to. Simply bail out before allocating memory.

Picked up by Coverity - CID 1222750.

Signed-off-by: Christoph Jaeger <christophjaeger@linux.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>