drm/amd/display: Drop config guard for DC_LOG_DP2

[Why & How]
It doesn't make sense to guard DC_LOG_DP2 by CONFIG_DRM_AMD_DCN, and
this also caused build failure for allmodconfig; So drop the guard
to fix the compile failure;

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>