drm: Use driver specified encoder name

Use the encoder name passed by the driver if non-NULL, otherwise fall
back to the old style name.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449592922-5545-5-git-send-email-ville.syrjala@linux.intel.com