drm/i915: don't use uninitialized EDID bpc values when picking pipe bpp

The EDID parser will zero out the bpc value, and the driver needs to handle
that case.  In our picker, we'll just ignore 0 values as far as bpp
picking goes.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=39323.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NKeith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/intel_display.c

@@ -4600,7 +4600,9 @@ static bool intel_choose_pipe_bpp_dither(struct drm_crtc *crtc,
 			if (connector->encoder != encoder)
 				continue;
 
-			if (connector->display_info.bpc < display_bpc) {
+			/* Don't use an invalid EDID bpc value */
+			if (connector->display_info.bpc &&
+			    connector->display_info.bpc < display_bpc) {
 				DRM_DEBUG_DRIVER("clamping display bpc (was %d) to EDID reported max of %d\n", display_bpc, connector->display_info.bpc);
 				display_bpc = connector->display_info.bpc;
 			}
@@ -5215,7 +5217,8 @@ static int ironlake_crtc_mode_set(struct drm_crtc *crtc,
 		temp |= PIPE_12BPC;
 		break;
 	default:
-		WARN(1, "intel_choose_pipe_bpp returned invalid value\n");
+		WARN(1, "intel_choose_pipe_bpp returned invalid value %d\n",
+			pipe_bpp);
 		temp |= PIPE_8BPC;
 		pipe_bpp = 24;
 		break;