drm/msm: Remove redundant null check

clk_prepare_enable() and clk_disable_unprepare() will check
NULL clock parameter, so It is not necessary to add additional checks.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>