msm: kgsl: Do not log an error if the L3 clock is not available

Currently l3_clk get failures are logged even if l3-pwrlevels
are not present. Fix this by getting the l3_clk at probe time.

Change-Id: Iacf15c6453223cb9d6434d4bd43adc5fcadcb002
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>