drm/i915: Remove intel_ prefix from encoder variables in intel_ddi.c

In I915 driver, there are many places where variable name for
intel_encoder object is given as 'intel_encoder' whereas it would
make more sense to call it just 'encoder' when possible.

This patch does this cleanup in file intel_ddi.c.

PS: There are few functions where both drm_encoder and intel_encoder
are present. For such functions, this patch does nothing.

Suggested-by: Ander Conselvan De Oliveira <conselvan2@gmail.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ander Conselvan De Oliveira <conselvan2@gmail.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489067021-4709-1-git-send-email-shashank.sharma@intel.com