drm/i915: Disable high-bpc on pre-1.4 EDID screens

Prevents black screens when using 30bpp framebuffers on my
HDMI screens here. The DP input on the same screen though reports a
1.4 EDID with the correct 8bpc limit set.

v2: Actually check for the right thing!
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7694,6 +7694,13 @@ pipe_config_set_bpp(struct drm_crtc *crtc,
 				      bpp, connector->display_info.bpc*3);
 			pipe_config->pipe_bpp = connector->display_info.bpc*3;
 		}
+
+		/* Clamp bpp to 8 on screens without EDID 1.4 */
+		if (connector->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;
+		}
 	}
 
 	return bpp;