msm: kgsl: get pm_qos_latency from device tree

pm_qos_latency parameter can be configured from
device tree as well. Will set to default value if
entry not found in target device tree.

Change-Id: I76271f4fdc26e191431e5dfe2a6de9e2b3ab426f
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>