mesa: remove len argument from _mesa_shader_debug()

mesa: remove len argument from _mesa_shader_debug()

There was only a single user which was using strlen(buf).
As this function is not user facing (i.e. we don't need to feed back
original length via a callback), we can simplify things.

Suggested-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>