amdgpu/dc: cleanup construct returns in gpio.

This is similiar to previous patches, don't return when we don't
need to, also do error checking before allocating memory, makes
it simpler to cleanup after.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>