drm/amdgpu/display: fix build error without CONFIG_DRM_AMD_DC_DSC_SUPPORT

If CONFIG_DRM_AMD_DC_DSC_SUPPORT is not set, build fails:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_hwseq.c: In function dcn20_hw_sequencer_construct:
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_hwseq.c:2099:28:
 error: dcn20_dsc_pg_control undeclared (first use in this function); did you mean dcn20_dpp_pg_control?
  dc->hwss.dsc_pg_control = dcn20_dsc_pg_control;
                            ^~~~~~~~~~~~~~~~~~~~
                            dcn20_dpp_pg_control

Use CONFIG_DRM_AMD_DC_DSC_SUPPORT to guard this.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 8a31820b ("drm/amd/display: Make init_hw and init_pipes generic for seamless boot")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>