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

include/drm/display/drm_dp.h

@@ -148,6 +148,7 @@
 #define DP_RECEIVE_PORT_0_CAP_0		    0x008
 # define DP_LOCAL_EDID_PRESENT		    (1 << 1)
 # define DP_ASSOCIATED_TO_PRECEDING_PORT    (1 << 2)
+# define DP_HBLANK_EXPANSION_CAPABLE        (1 << 3)
 
 #define DP_RECEIVE_PORT_0_BUFFER_SIZE	    0x009
 
@@ -699,6 +700,7 @@
 
 #define DP_DSC_ENABLE                       0x160   /* DP 1.4 */
 # define DP_DECOMPRESSION_EN                (1 << 0)
+# define DP_DSC_PASSTHROUGH_EN		    (1 << 1)
 #define DP_DSC_CONFIGURATION				0x161	/* DP 2.0 */
 
 #define DP_PSR_EN_CFG				0x170   /* XXX 1.2? */