drm: Use IS_ERR() instead of IS_ERR_OR_NULL()

dev_pm_opp_find_freq_exact never return null but ERR_PTR(),so we
should use IS_ERR() to clarify and avoid compilation warnings.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Link: https://lore.kernel.org/r/1604630714-6163-1-git-send-email-wangqing@vivo.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>