drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"

This reverts commit 013dd9e03872
("drm/i915/dp: fall back to 18 bpp when sink capability is unknown")

This commit introduced a regression into stable kernels,
as it reduces output color depth to 6 bpc for any video
sink connected to a Displayport connector if that sink
doesn't report a specific color depth via EDID, or if
our EDID parser doesn't actually recognize the proper
bpc from EDID.

Affected are active DisplayPort->VGA converters and
active DisplayPort->DVI converters. Both should be
able to handle 8 bpc, but are degraded to 6 bpc with
this patch.

The reverted commit was meant to fix
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=105331

A followup patch implements a fix for that specific bug,
which is caused by a faulty EDID of the affected DP panel
by adding a new EDID quirk for that panel.

DP 18 bpp fallback handling and other improvements to
DP sink bpc detection will be handled for future
kernels in a separate series of patches.

Please backport to stable.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: stable@vger.kernel.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/intel_display.c

@@ -12106,21 +12106,11 @@ connected_sink_compute_bpp(struct intel_connector *connector,
 		pipe_config->pipe_bpp = connector->base.display_info.bpc*3;
 	}
 
-	/* Clamp bpp to default limit on screens without EDID 1.4 */
-	if (connector->base.display_info.bpc == 0) {
-		int type = connector->base.connector_type;
-		int clamp_bpp = 24;
-
-		/* Fall back to 18 bpp when DP sink capability is unknown. */
-		if (type == DRM_MODE_CONNECTOR_DisplayPort ||
-		    type == DRM_MODE_CONNECTOR_eDP)
-			clamp_bpp = 18;
-
-		if (bpp > clamp_bpp) {
-			DRM_DEBUG_KMS("clamping display bpp (was %d) to default limit of %d\n",
-				      bpp, clamp_bpp);
-			pipe_config->pipe_bpp = clamp_bpp;
-		}
+	/* Clamp bpp to 8 on screens without EDID 1.4 */
+	if (connector->base.display_info.bpc == 0 && bpp > 24) {
+		DRM_DEBUG_KMS("clamping display bpp (was %d) to default limit of 24\n",
+			      bpp);
+		pipe_config->pipe_bpp = 24;
 	}
 }