msm: kgsl: Get RGMU firmware version from register

The RGMU firmware writes the version information to
GMU general register. Read firmware version from
the register and store it to rgmu structure.

Change-Id: I1dcc8d7e606e486ba81702d824419b84f71b2c1f
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>