drm/amd/display: Return directly after a failed kzalloc() in dc_create()

* Return directly after a call of the function “kzalloc” failed
  at the beginning.

* Delete a label which became unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>