qxl: Allow resolution which are not multiple of 8

The use of drm_cvt_mode() in qxl_add_monitors_config_modes() means that
the resolutions we are going to present to user-space are going to be
rounded down to a multiple of 8. In the QXL arbitrary resolution case,
this is not useful.
This commit forces the actual width/height that was requested by the
client in the drm_display_mode structure rather than keeping the
rounded version.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Message-id: 20161108091209.25568-8-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>