glamor: fix glamor_xv_query_image_attributes() for odd-width images

Images which are one pixel wider than a multiple of 8 are being handled
incorrectly. Other drivers round up the width to a multiple of two
before they start calculating. Do the same.

https://bugzilla.gnome.org/show_bug.cgi?id=795235



Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>