i915: Use L8A8 instead of I8 to simulate A8 on gen2

i915: Use L8A8 instead of I8 to simulate A8 on gen2

Gen2 doesn't support the A8 texture format. Currently the driver
substitutes it with I8, but that results in incorrect RGB values.
Use A8L8 instead. We end up wasting a bit of memory, but at least
we should get the correct results.

v2: Handle the fallback in _mesa_choose_tex_format() and also
    do it for all alpha formats that currently accept A8

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72819
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80050
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38873
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>