drm/i915: update DSC feature flag handling during device init

DSC feature information is no longer part of the DFSM register in
some display generations.

Bspec:50075
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011093048.447177-1-vinod.govindapillai@intel.com