arm/dt: msm8974: change mdp reserved memory for liquid

Due to larger panel size in Liquid devices, more memory needs to be
reserved for display frame buffer, the new buffer size is required in
order to hold two 1080p ARGB8888 buffers.

Change-Id: Ib22b867cb89ad81888f44ecbf6db32265c48c2dc
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>