msm: Don't allow overmapping

Due to the way allocations are performed, over mapping must be
disallowed. Otherwise, it may be possible to inadvertently
unmap other physical addresses. The smallest length that can
now be mapped is 4K. Lengths less than 4k can be safely
overmapped since physical addresses must be 4k aligned as well.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>