softpipe: don't ignore pipe_constant_buffer::buffer_offset

softpipe: don't ignore pipe_constant_buffer::buffer_offset

This was never a problem since the Mesa state tracker always gives
us a user-space constant buffer with buffer_offset=0.  But if another
state tracker ever gave us a "HW" constant buffer with non-zero
buffer_offset we'd mis-render.

Also, use the correct buffer size.  And move an assertion to the
top of the function.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>