msm: mdss: Fix invalid dma attachment during fb shutdown

If DMA attachment fail during fb_mmap, all ION memory will get free. It
is necessary to reset the fbmem and fb_attachemnt pointer to NULL,
otherwise during shutdown will perform another free and causing issue.

CRs-Fixed: 1090244
Change-Id: I92affcf2ce039eecfc72b7c191e058f37815c726
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>