drm/amd/display: fix undefined struct member reference

An initialization was added for two optional struct members.  One of
these is always present in the dcn20_resource file, but the other one
depends on CONFIG_DRM_AMD_DC_DSC_SUPPORT and causes a build failure if
that is missing:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_resource.c:926:14: error: excess elements in struct initializer [-Werror]
   .num_dsc = 5,

Add another #ifdef around the assignment.

Fixes: c3d03c5a ("drm/amd/display: Include num_vmid and num_dsc within NV14's resource caps")
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>