drm/i915: make transcoder_name return a string

Nicer for eDP (actually "EDP" instead of "D"), and makes future
expansion for DSI transcoders easier.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/2ac55ed584e450fe154daecb6453dff8eede5e7d.1458070700.git.jani.nikula@intel.com