msm: kgsl: Mask the number of registers in the register protection span

Make sure to mask the size of the span for protected registers to avoid
accidently messing up the rest of the bit field.  Also, set the start
and end offsets the same for the infinity span so we don't write an
underflow value to the register.

Change-Id: Ic0dedbad7eedcc6cf6cc50889f91c4b068deef2f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>