drm/i915: Add debug message for FB plane[0].offset!=0 error

Print a debug message if the FB plane[0] offset is not 0 as expected, to
help understainding an add FB IOCTL fail.

Cc: Chris Wilson <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: Mika Kahola <[e-mail]>
Signed-off-by: Imre Deak <[e-mail]>
Reviewed-by: Mika Kahola <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/display/intel_display.c

···
 	}
 
 	/* FIXME need to adjust LINOFF/TILEOFF accordingly. */
-	if (mode_cmd->offsets[0] != 0)
+	if (mode_cmd->offsets[0] != 0) {
+		DRM_DEBUG_KMS("plane 0 offset (0x%08x) must be 0\n",
+			      mode_cmd->offsets[0]);
 		goto err;
+	}
 
 	drm_helper_mode_fill_fb_struct(&dev_priv->drm, fb, mode_cmd);