drm/amd/display: Move dml code under CONFIG_DRM_AMD_DC_FP guard

For some reason, the dml code is not guarded under CONFIG_DRM_AMD_DC_FP
in the Makefile. This commit moves the dml code under the DC_FP guard.

Reviewed-by: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>