drm/msm/dpu: dev_pm_opp_put_clkname() only when an opp_table exists

Its possible that dpu_bind() fails early enough before
dev_pm_opp_set_clkname() is called. In such cases, unconditionally
calling dev_pm_opp_put_clkname() in dpu_unbind() can result in
a crash. Put an additional check so that dev_pm_opp_put_clkname()
is called only when an opp_table exists.

Fixes: aa3950767d05 ("drm/msm/dpu: Use OPP API to set clk/perf state")
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>