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: 8c2f14c36f47ce ("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>