drm/amd/display: fix warning: ‘update_dsc_caps’ and ‘apply_dsc_policy_for_stream’ defined but not used

Fixes gcc '-Wunused-function' warning:

‘update_dsc_caps’ and ‘apply_dsc_policy_for_stream’ are only used
if 'CONFIG_DRM_AMD_DC_DCN' is defined,

however, it's defined even if 'CONFIG_DRM_AMD_DC_DCN' is not defined.
Thus gcc will report following warning
if 'CONFIG_DRM_AMD_DC_DCN' is not defined:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5572:13: warning:
‘apply_dsc_policy_for_stream’ defined but not used [-Wunused-function]

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5556:13: warning:
‘update_dsc_caps’ defined but not used [-Wunused-function]

Thus move the definition of ‘update_dsc_caps’ and
‘apply_dsc_policy_for_stream’ inside define macro to fix it.
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>