drm/amdgpu/display: properly guard dc_dsc_stream_bandwidth_in_kbps

Move the function protoype to the right header and guard
the call with CONFIG_DRM_AMD_DC_DCN as DSC is only available
with DCN.

Fixes: a03f6c0e ("drm/amd/display: Add changes for dsc bpp in 16ths and unify bw calculations")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Dillon Varone <dillon.varone@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>