msm: kgsl: Fix NULL pointer dereference

Before dereferencing the register base, check
if it has been initialized or not. This saves us
a device crash during gmu snapshot that occurs
before the register base is initialized.

Change-Id: I5402a82b0549b219f75f9268724f7a114aaeddf0
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>