svga: fix PIPE_CAP_MAX_TEXTURE_BUFFER_SIZE value

svga: fix PIPE_CAP_MAX_TEXTURE_BUFFER_SIZE value

This query is supposed to return the max texture buffer size/width in
texels, not size in bytes.  Divide by 16 (the largest format size) to
return texels.

Fixes Piglit arb_texture_buffer_object-max-size test.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by :Charmaine Lee <charmainel@vmware.com>