i965: expose RGBA visuals only on Android

i965: expose RGBA visuals only on Android

As Marek pointed out in earlier commit - exposing RGBA on other
platforms introduces ~500 Visuals, which are not tested.

Note that this does not quite happen, yet. Reason being that the GLX
code does not check the masks - see scaralEqual().

Thus as we fix that, we'll run into the issue described.

v2: Rebase, while keeping loaderPrivate
v3: Beef-up commit message, getCapability() returns unsigned (Tapani)

Fixes: 1bf703e4ea5 ("dri_interface,egl,gallium: only expose RGBA visuals
on Android")
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Chad Versace <chadversary@chromium.org>
Cc: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>