drm/amdgpu: Fix undefined dm_ip_block for navi12

There is missing "if defined" CONFIG_DRM_AMD_DC block for non DC
configurations. This will cause link error. The patch is fixing that.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=110979


Signed-off-by: Petr Cvek <petrcvekcz@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>