drm/dp: Add DP_HBLANK_EXPANSION_CAPABLE and DSC_PASSTHROUGH_EN DPCD flags

Add the DPCD flag to enable DSC passthrough in a last branch device,
used in a follow-up i915 patch.

Also add a flag to detect HBLANK expansion support in a branch device,
used by a workaround in a follow-up i915 patch.

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-8-imre.deak@intel.com