drm/dp: use more of the extended receiver cap

Extend the use of extended receiver cap at 0x2200 to cover
MAIN_LINK_CHANNEL_CODING_CAP in 0x2206, in case an implementation hides
the DP 2.0 128b/132b channel encoding cap.

v2: Extend to DP_RECEIVER_CAP_SIZE (Ville)

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/649051cb896821147feee91aab1f2abc523c1353.1631191763.git.jani.nikula@intel.com

drivers/gpu/drm/drm_dp_helper.c

@@ -608,7 +608,7 @@ static u8 drm_dp_downstream_port_count(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 static int drm_dp_read_extended_dpcd_caps(struct drm_dp_aux *aux,
 					  u8 dpcd[DP_RECEIVER_CAP_SIZE])
 {
-	u8 dpcd_ext[6];
+	u8 dpcd_ext[DP_RECEIVER_CAP_SIZE];
 	int ret;
 
 	/*