msm: kgsl: Make sure WPTR reg is updated properly

Sometimes writes to WPTR register do not take effect, causing a
3D core hang. Make sure the WPTR is updated properly when waiting.

Change-Id: I86559198c65a1f5ae8542a479daef054847e5158
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>

Conflicts:

	drivers/gpu/msm/adreno.c

CRs-fixed: 293110
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>