msm: kgsl: Clear halt before gmu boot

During hard reset if we are not able to toggle the CX gdsc,
the halt will remain asserted at the time of next GMU boot.
This causes GMU boot to fail and lead to NoC errors. So clear
the halt bit before attempting GMU boot. Also, move the wptr
retention out among the rest of the register writes instead of
piling them inside a6xx_gmu_start().

Change-Id: Id214e0a9261c1f14de13864c715cf1a9d415e1e7
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>