gallium/radeon/winsyses: set reasonable max_alloc_size

which is returned for GL_MAX_TEXTURE_BUFFER_SIZE.
It doesn't have any other use at the moment.
Bigger allocations are not rejected.

This fixes GL45-CTS.texture_buffer.texture_buffer_max_size on Bonaire.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
2 files changed