egl/x11: pick the user requested screen

egl/x11: pick the user requested screen

At the moment the user will pass the screen number via attribs, yet we
would throw that away. Reason being that the int *screen passed to
xcb_connect() is output only.

v2 (Emil):
 - split from a larger patch
 - use xcb_connect() returned screen, as fallback
 - use helper function only as needed

Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>