mdss: fb: Add backward compatibility to fb memory

In the current implementation, contiguous memory allocation
during bootup is removed and related changes were made in
frame buffer driver which is not backward compatible. With this
change, add backward compatibility to map to physically contiguous
memory when available which is allocated at bootup.

Change-Id: Ib1a595f2426dda8d3e1c5e50c71c35d3f45854b6
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>