tdm_hwc: add tdm_hwc_get_release_fences interface

tdm_hwc: add tdm_hwc_get_release_fences interface

Retrieves the windows which the backend requires setting the release fences
the release fence is signaled when the backend is no longer using previous buffer.

tdm_error tdm_hwc_get_release_fences(tdm_hwc *hwc, uint32_t *num_elements,
			tdm_hwc_window **hwc_windows, int *fences);
tdm_error (*tdm_hwc_get_release_fences)(tdm_hwc *hwc, uint32_t *num_elements,
			tdm_hwc_window **hwc_windows, int *release_fences);

Change-Id: Ie8bd337c6da2c49368263c701f2dcdcc31655f62