msm: kgsl: Scale hub clock to 150 Mhz

This is a new clock in a660 which needs to be run at
150 Mhz instead of running at 19.2 Mhz by default.

Also, there is no need to store a gmu_clk pointer
so get rid of it too.

Change-Id: If2741c061339e914683ca25565d1bac3ec6ac100
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>