dma-buf: heaps: Use _IOCTL_ for userspace IOCTL identifier

This is more consistent with the DMA and DRM frameworks convention. This
patch is only a name change, no logic is changed.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216133405.1001-2-afd@ti.com