i965: Do not generate D16 B5G6R5_UNORM configs on gen < 8

i965: Do not generate D16 B5G6R5_UNORM configs on gen < 8

We don't support MESA_FORMAT_Z_UNORM16 before Gen8, see
intel_screen_init_surface_formats.

As a consequence disables B5G6R5_UNORM configs with depth
on gen < 6.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2275
CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3206>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3206>