msm: mdss: clear fbmem_buf on ion_free

Even after ion_free, fbmem_buf is not cleared and contains
some random address. fbmem_buf is assumed to be mapped, when it
is not NULL, which leads to issues. Assign NULL to fbmem_buf, as soon
as ion_free is done.

Change-Id: I712f64a31c8a555f793ff2dd5fc1dea4771c8d0e
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>