fsl-diu-fb: Pass the proper device for dma mapping routines

The driver should pass a device that specifies internal DMA ops, but
currently NULL pointers are passed, and thus following bug pops up:

  Freescale DIU driver
  ------------[ cut here ]------------
  kernel BUG at arch/powerpc/include/asm/dma-mapping.h:237!
  Oops: Exception in kernel mode, sig: 5 [#1]
  ...
  NIP [c01658b4] allocate_buf+0x0/0x8
  LR [c0306554] fsl_diu_probe+0x2b4/0x518
  Call Trace:
  [df02be10] [c030638c] fsl_diu_probe+0xec/0x518 (unreliable)
  [df02be60] [c020cdec] of_platform_device_probe+0x5c/0x84
  [df02be80] [c018f5d0] really_probe+0x78/0x1a0
  [df02bea0] [c018f7c0] __driver_attach+0xa4/0xa8
  [df02bec0] [c018ea00] bus_for_each_dev+0x60/0x9c
  [df02bef0] [c018f414] driver_attach+0x24/0x34
  [df02bf00] [c018f168] bus_add_driver+0x12c/0x1cc
  [df02bf20] [c018fbdc] driver_register+0x6c/0x110
  [df02bf30] [c020ccb4] of_register_driver+0x54/0x70
  [df02bf40] [c03d0a50] fsl_diu_init+0x70/0xa4
  ...

This patch fixes the issue.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>