drm/amd/display: Fix "dc has no member named dml" compile error

For DCN disabled builds, dc->dml is stripped out. Therefore, guard usage
in dc_create_state() with CONFIG_DRM_AMD_DC_DCN1_0.

It fixes the following error:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c: In function 'dc_create_state':
>> drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:1237:34: error: 'struct dc' has no member named 'dml'
     memcpy(&context->bw_ctx.dml, &dc->dml, sizeof(struct display_mode_lib));
                                     ^~

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>