BACKPORT: 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
Bug: 36251984
Change-Id: I36eb3ca12ae90789b2b63ddd6d9ed69517b2eb59
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>
Signed-off-by: Jonathan Solnit <jsolnit@google.com>