drm/i915/dmabuf: Update dma_buf_ops.unmap_dma_buf callback to use drm_gem_unmap_dma_buf()

The dma_buf_ops.unmap_dma_buf callback used in i915,
i915_gem_unmap_dma_buf(), has the same code as drm_gem_unmap_dma_buf().
In order to eliminate defining and using duplicate function, it updates
the dma_buf_ops.unmap_dma_buf callback to use drm_gem_unmap_dma_buf().

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225131316.1433515-2-gwan-gyeong.mun@intel.com