st/nine: Clamp GetAvailableTextureMem

st/nine: Clamp GetAvailableTextureMem

Previously we used to clamp "available_texture_limit",
which was incorrect. "available_texture_mem" should
have been clamped instead.
The resulting code was noop.

The idea behind that code was that 32 bits executable
would see maximum 4GB video memory.

However it seems according to users that 32 bits apps
should be able to allocate more than 4GB, thus the
clamping is inappropriate.

Instead clamp the return of GetAvailableTextureMem, to
correctly report a high value when there is more than
4GB available.

I do not know what should exactly be the clamp value,
for now have a 64MB margin below UINT_MAX.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Acked-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9177>