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

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

BugLink: https://bugs.launchpad.net/bugs/1849576
commit 5fb9b797d5ccf311ae4aba69e86080d47668b5f7 upstream.

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: c4d8cfe516dc ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>