i965/vs: Fix debug dumping of VS push constants.

i965/vs: Fix debug dumping of VS push constants.

While copying the values into the batch space, we advance the param
pointer.  The debug code then tries to iterate over all the uploaded
values, starting at param...which is now the end of the uploaded data,
rather than the start.

This patch saves a pointer to the start of push constant space before
it gets altered and switches the debug code to use that.

Tested by uncommenting the code and examining the output of
glsl-vs-clamp-1.shader_test.  Previously all values appeared to be zero.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>