drm/amd/display: Fix POWERPC_64 compilation

[Why]
Compilation errors while compiling without CONFIG_DRM_AMD_DC_FP:
"undefined reference to `dc_bandwidth_in_kbps_from_timing'"

[How]
Fix Makefile to move dsc files out of DC_FP guard.

Fixes: 00c39110 ("drm/amd/display: Add misc DC changes for DCN401")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>