msm: mdss: modify watermark levels when horizontal flip is present

When horizontal flip is present on a particular pipe, one line out of
the shared memory pool is reserved for consuming the line in reverse
direction. When this happens the consumed line does not account in the
watermark threshold and hence the priority levels being programmed would
be based on the remaining memory in which buffer contents are being
fetched. In order to have proper priority mappings, update the watermark
levels to account for this behavior

Change-Id: I80cebccf0083b2e9edcf3bf73818fde160ef9c63
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>