drm/i915: check for return value

We were not checking the return value of drm_encoder_init() which can
fail. And if it fails then we will be working with an uninitialized
encoder.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1444312681-10149-2-git-send-email-sudipm.mukherjee@gmail.com