msm_fb: display: fix underrun seen with big src coordinates

MDP underrun happens if combination of x + w or y + h exceeds 2048.
To handle such use cases add an offset to the source address instead.

Change-Id: I36a8b687c4e21cac9915b5bf71211c97f6b44ddd
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>