arm/dt: 8974: change MDP QoS priority remapings

Change the priority remapping of MDP to properly use water mark levels
to request higher priority. Since 8974v1 settings should be the same
as v2, keep only single entry which is common for both.

Change-Id: Ie78323d57eebf39f6b8883ea35ea81bf53a0ebc6
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>