msm: mdss: refactor ot limits to support rd and wr

Current code only supports to configure the same
read and write ot limits. For newer platforms it is
needed to configure different default limits for
each of the operations. This patch refactors the
implementation so the driver can support a different
default limit for the read and the write operations.

Change-Id: Ie54e4f24d52a561a6bf10de65ab6bce8a98211bb
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>