drm/bochs: add edid present check

Check header before trying to read the complete edid blob, to avoid the
log being spammed in case qemu has no edid support (old qemu or edid
support turned off).

Fixes: 01f23459 drm/bochs: add edid support.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181220101122.16153-1-kraxel@redhat.com

drivers/gpu/drm/bochs/bochs_hw.c

@@ -86,9 +86,16 @@ static int bochs_get_edid_block(void *data, u8 *buf,
 
 int bochs_hw_load_edid(struct bochs_device *bochs)
 {
+	u8 header[8];
+
 	if (!bochs->mmio)
 		return -1;
 
+	/* check header to detect whenever edid support is enabled in qemu */
+	bochs_get_edid_block(bochs, header, 0, ARRAY_SIZE(header));
+	if (drm_edid_header_is_valid(header) != 8)
+		return -1;
+
 	kfree(bochs->edid);
 	bochs->edid = drm_do_get_edid(&bochs->connector,
 				      bochs_get_edid_block, bochs);