msm: kgsl: Update GPU idleness check when RB is empty

GPU is considered to be idle when ring buffer is idle in current design.
But ring buffer empty will not alone determine GPU idleness. RBBM
status also should be checked along with ring buffer status to determine
GPU idleness.

Change-Id: I165edfb7f10144612525f240817fd77ac5da6efb
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>