msm: kgsl: Add out of bound check for gmu register access

Read/write operation to out of gmu register address space
will not give the expected results. Add a check to allow the
access to gmu registers only.

Change-Id: Ifb88c1966137531daa5c7a58c70a9bdf0a584b23
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>