nine: correctly divide maximum constant numbers

We get from driver number already multiplied by sizeof(float[4]) = 16.
This patch shouldn't affect any modern card with
PIPE_SHADER_CAP_MAX_CONST_BUFFER_SIZE > 256

Reported-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>