mdss: display: allocate buffer from ion when debug writeback enabled

Debug writeback is used to output MDSS framebuffer contents to memory to
for debugging of MDSS core. Use ion buffer allocation for debug
writeback buffer instead of using memory from framebuffer base layer to
allow entire framebuffer area to be used for composition.

Change-Id: Ia1a91f5f14bed1d29691092142fc94f7a8b6f4d2
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>