drm/msm/dsi: Fix return value check for clk_get_parent

drm/msm/dsi: Fix return value check for clk_get_parent

clk_get_parent returns an error pointer upon failure, not NULL. So the
checks as they exist won't catch a failure. This patch changes the
checks and the return values to properly handle an error pointer.

Fixes: 60f891478e4a ("drm/msm/dsi: add implementation for helper functions")
Cc: Sibi Sankar <sibis@codeaurora.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Rob Clark <robdclark@chromium.org>
Cc: <stable@vger.kernel.org> # v4.19+
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>