drm/lima: Fix opp clkname setting in case of missing regulator

Commit d8c32d39 ("drm/lima: Migrate to dev_pm_opp_set_config()")
introduced a regression as it may undo the clk_names setting in case
the optional regulator is missing. This resulted in test and performance
regressions with lima.

Restore the old behavior where clk_names is set separately so it is not
undone in case of a missing optional regulator.

Fixes: d8c32d39

 ("drm/lima: Migrate to dev_pm_opp_set_config()")
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221027073200.3885839-1-nunes.erico@gmail.com