drm: replace drm_get_encoder_name() with direct name field use

Generated using semantic patch:

@@
expression E;
@@

- drm_get_encoder_name(E)
+ E->name

Acked-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>