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