msm: kgsl: Add a check for valid frequency of RBBM timer clock

rbbmtimer_clk hardwired fixed XO frequency 19.2Mhz. clock_round_rate()
for RBBM timer clock fails with -EPERM as it is fixed. So added a
check to make sure that rbbmtimer_clk clk_set_rate() has valid
frequency. This avoids warning message in the driver log.

Change-Id: I8f8bcec88e6a39e1550bb67590e6b66dba8e7a27
Signed-off-by: Archana Obannagari <aobann@codeaurora.org>