drm/cma: Remove call to drm_gem_free_mmap_offset()

drm_gem_object_release() called later in the drm_gem_cma_free_object()
function already calls this, so there's no need to do this explicitly.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>