drm/i915: Generalize transcoder looping

drm/i915: Generalize transcoder looping

To make looping through transcoders in intel_ddi.c more generic, let's switch
to use 'for_each_pipe()' macro to do this.

v2: Add a notion that we are dealing with transcoders instead of pipes (Jani)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1510216670-16848-1-git-send-email-mika.kahola@intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>