drm/omap: Fix runtime PM imbalance on error

pm_runtime_get_sync() increments the runtime PM usage counter
even when it returns an error code. However, users of its
direct wrappers in omapdrm assume that PM usage counter will
not change on error. Thus a pairing decrement is needed on
the error handling path for these wrappers to keep the counter
balanced.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200822065743.13671-1-dinghao.liu@zju.edu.cn