msm: kgsl: A430 has more protected mode registers available

A430 extends the number of protected mode registers from 16 to 24.
Update the BUG_ON() restriction to give developers more freedom
to save ourselves from badly behaving user mode drivers.

Change-Id: Ic0dedbad9bcb42e80d0380547260917f58514699
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>