msm: kgsl: Add a check for availability of RBBM timer clock

Before calling kgsl_pwrctrl_clk_set_rate(), make sure the
RBBM timer clock is available. This avoids warning messages
in the driver log.

Change-Id: I0cd21b7253c802e8522d570056c8aeda02729267
Signed-off-by: Archana Obannagari <aobann@codeaurora.org>