drm: shmob_drm: Check clk_prepare_enable() return value

The clk_prepare_enable() call can fail. Check it's return value. We
can't propagate it all the way to the user as the KMS operations in
which the clock is enabled return a void.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>