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: If998615655f69d9d867d7655d617083d3d9c03eb
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>
Signed-off-by: Jonathan Solnit <jsolnit@google.com>