kgsl: msm: Correctly check hardware idle status for a619_holi target

Use GMU_AO_GPU_CX_BUSY_STATUS instead of RBBM_STATUS to check the
hardware idle status for A619_holi target.

Change-Id: Ic66e80924331d9f1a0203fd96e352c3009a4eb75
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>