mdss: msm_fb: Allocate fb memory during mmap

Current design of display driver (overlay) is no more
using the physically contiguous memory for normal display
functionality.

However, we still need to be backward compatible for any client
using the legacy implementation (eg. pan display) functionality.
As soon as there is a mmap call, the allocation happens from
system heap and gets removed once the references to frame buffer
driver becomes NULL.

With this change one can remove the physically contiguous memory
which gets allocated at the boot time and the usage of physically
contiguous memory is optimized.

Change-Id: I1b6040187bda8095007faa4d9df7dc046aa30c6e
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>